Can you prevent being reseated at a middle seat in economy, after a change of airplane model?

I will assume that when you say "almost phobic" you mean you have an actual condition that needs some sort of accommodation. If not, this answer stands except for the parts where you tell the staff you have a phobia. Don't make up a condition because you don't like the middle seat. You asked if there is anything you can do to prevent getting moved to a middle seat? Yes, there is, though nothing is 100% effective.

Let's think about how a change of equipment would move you to a middle seat. First, consider the case where the plane they planned to use breaks in some way. They look around for another plane to use. If it's the same model, there is no issue and you get the same seat. If it's a larger model, there is no issue. All the seats that existed before will continue to exist and everyone will get the same seat.

Where the trouble happens is when it's a smaller model, or it's smaller in at least one of the two "sections" for a two-class plane. They can only use this smaller plane if the flight is quite underbooked. Say it had 30 rows in economy and nobody was in a middle seat. They can use a 20-row plane, but 10 rows worth of people are moving to a middle seat. Yuck. Alternatively, maybe 10 people from business class have to be moved back to economy, and they are going to be put in the best possible seats, meaning that someone who had one of those seats is moving.

Your best protection here is to be highest up the totem pole when the shuffling starts to happen. In order of usefulness, try:

• have status with the airline. Be an "Elite" or whatever they call it - the folks who get to board first
• be travelling alone
• check in early
• speak to the humans at the gate before the trouble happens (which means your habit needs to be to do this every time) and just confirm your seat, and tell them you have a phobia which may give you panic attacks if you are moved from your (window or aisle, whichever you have) seat. DO NOT say you have a phobia of the middle seat. It will be taken as a joke. When they confirm your seat, don't say anything grumpy or entitled, give them a big smile and thank them and tell them you're relieved that the seat selection process works so well.

Now consider the second possibility: something has gone wrong up the line and they're getting a bigger plane and merging two flights, or accommodating a bunch of people from some similar flight. The people from the messed up flight all lose their seats, but some of them have status or were in business class on the other flight and may push you out of your seat. What can you do? The same set of strategies, but with less chance of success.

When you get your new boarding pass, react immediately. If you care this strongly about it, you need to KNOW that 17B is a middle seat, not worry and wonder and find out when you go to sit down. It's easy in the low letters of the alphabet, but 22I is a little harder to place, isn't it? If necessary, ask as soon as you are given the pass. "22I? Is that a window seat?" When you're told it's a middle, react immediately. "I'm sorry, I know nobody likes the middle, but I will have a panic attack if I am not at the window. I will sit further back if I have to, is there any way you can put me against the window please?"

Having said this, don't you trot all this out just because you don't like it. Nobody likes it. Your meticulous planning may let you down (unless it includes flying 100,000 miles a year on the same airline alliance) because stuff happens some days, and you may just have to suck it up. But if you really are going to freak out, tell them while you're still holding the boarding pass and there's something they can do about it. And before you do, know your answer to "I can move you to the flight that leaves in 3 hours and put you in 12A; will that work?"

^{@KateGregory has handled the title question quite well, so I'm just responding to the question in the body of your post.}

You could probably string together a series of Greek or Latin root words and call it a new phobia if you want (I'm guessing this is how Coriat came up with "triskaidekaphobia", and that he was not alone in using that method)...but first of all, phobias are usually irrational, and given your history, your aversion to the middle seat does not sound irrational. (And you have my sympathy!) I always avoid middle seats too; I usually look for the most isolated seat I can find, preferably toward the back so everybody else's loud mouths are pointed away from me. Nothing irrational about this. People are annoying (and I somehow got my doctorate studying them anyway).

Second, if you think you really do have an intolerable reaction to this situation and can't keep it together if you find yourself there, it's possible you do have something personal to worry about. All else being equal, it's not fair for you to request special treatment, because chances are very few people wouldn't rather sit somewhere else too, and we can't all avoid the middle seats of course. If this doesn't address your reason for asking, maybe it would be worthwhile to talk to a psychiatrist. I'm not one myself – I'm just a psychologist specializing in ordinary personality variation – so I can't say that it's impossible for you to have a specific, legitimate phobia about this, or that it's impossible for you to develop some form of posttraumatic stress or social anxiety disorder just due to your legitimately awful experiences. It doesn't seem likely, but I can say that vulnerability to developing a disorder varies across individuals, so arguably no one can rule it out completely (see the diathesis-stress model for more on this principle).

Of course, there's little or no harm in seeing a therapist one way or another. They're not going to haul you off in a white straightjacket or force-feed you pills, and if they recommend any, you can always refuse (and you should always study any medication you're prescribed for yourself, IMO). Everyone can benefit from a little counseling, including normal, healthy people, which might very well include you (I'd like to say it probably does, but I don't want to go anywhere near a personal diagnosis in this answer). If you have insurance, the visit will probably be covered.

Aside from PTSD and social anxiety, there are other possibilities that would be nice to rule out (e.g., agoraphobia). This is the benefit of actually going to see a professional about it: you can get a clean bill of mental health if you really are completely healthy, and maybe even some useful tips if you're not quite there but definitely subclinical. Therapists are full of useful strategies for managing emotions that everyone could probably stand to know more about, disordered or not.

Forgive me for ending on a relatively grim note, but I wouldn't want to be accused of not taking your concern seriously. Until you see a professional, you also can't really rule out the possibility of a real disorder affecting you. This isn't to say, "Everyone and their mother should go get checked out right away, because you never know!" but if you've got some tangible, concerning signs, don't ignore them – you wouldn't ignore a physical symptom of something serious potentially affecting your heart, for instance, so why be any more cavalier about your brain? Again, I don't hear anything that sounds truly diagnostic or threatening here, but I'm not qualified to give you a clean bill either. Some sites on the internet will try, but whether they should is debatable (see "Finding help with psychological and emotional problems" on Meta CogSci), and I wouldn't want to go there even if I was qualified.

Best to see a professional in person if you feel there's a serious risk of panic attack or any other kind of unusual harm or loss of control afflicting you if you find yourself in the middle seat again. As @NateEldredge's comment indicates, you could seriously disrupt the operation of the flight if you let yourself get too out-of-sorts. Of course, if you do have special psychological conditions that make this situation exceptionally difficult for you, this is not an "all else being equal" kind of scenario, and you have every right to request special accommodation. Whether you'll be granted it is another question of course (or at least a part of yours that I can't answer)...but (this might be irresponsible of me to say) so is whether they'd actually do anything to verify it if you claimed to have a special vulnerability. I certainly hope they would take you seriously...but people with social anxiety disorders don't walk around with cards in their wallets for the sake of proving it to people. If you do get a positive diagnosis at any point, you might want to ask for special documentation just so you don't have to worry about being dismissed outright. If you don't get diagnosed (which sometimes happens even to people who should), at least you know some abnormal psych jargon you can throw at a flight attendant to get your point across.