What happens to an existing TSA Precheck registration if I apply for Global Entry?

Global Entry provides all the benefits of TSA Precheck (referencing the comparison chart Dorothy pointed out). In particular the CBP ID that is assigned as your Global Entry membership number can be used as the Known Traveler Number for airline reservations to get TSA Pre encoded on your boarding passes in exactly the same way you use the TSA Precheck ID number now. Since Global Entry provides this plus expedited processing through US immigration (and, if you carry the card, access to NEXUS lines though security in Canadian airports, though not the NEXUS machines at Canadian immigration) I suspect that going forward you'll probably want to let the TSA Precheck membership expire and maintain Global Entry alone.

While I'm not sure how interdependent they are, note that TSA Precheck and Global Entry applications are made through separate web portals, suggesting they aren't closely integrated. All Global Entry/NEXUS/Sentri ID numbers I have seen have "98" as their first two digits so if your current TSA Precheck number does not start with "98" I would also assume the Global Entry membership will come with its own Known Traveller Number. As the TSA Precheck FAQ has a question about having more than one KTN with an answer starting with

You may use any active KTN that you have been assigned.

my guess is that your current TSA Precheck membership will continue on unaffected by the GE membership (how else could you have more than one KTN?) and that either KTN will work for airline reservations for as long as both memberships remain valid.

This obviously involves speculation, though, since I've not done this. When you find out the right answer please report back!

What happens to my TSA Precheck ID if I proceed with my application?

Nothing. You will still be PreCheck.

Does applying for Global Entry affect my existing TSA Precheck in any way?

It might change why you have PreCheck, through Global Entry vs TSA application, but it doesn't matter, you're now PreCheck and Global Entry.

Will I end up with two IDs?

You will receive a Global Entry card which is unrelated to PreCheck.

Are there any issues with adding Global Entry when I already have TSA Precheck?

No.

I obtained my GE number and found I was no longer getting precheck status. I called airline, and they had me change my KTN from precheck number to the GE number. Hopefully that does that job.