French Strikes: Using idTGV ticket for standard (SNCF) TGV

Ok, so the reality uncovered:

1. You can take any train it seems, at least on board of both trains I took they said that any ticket for this or the previous day is valid. However, nobody checked anyone in any of these trains, and I got similar experiences from other people I met in the last couple days. At all the stations, the boarding was a complete chaos; the SNCF staff only made sure that people don't fall down on the other side of the platform, that they spread themselves regularly and that they get on a train to the correct destination. Yet, there seem to be trains where ticket expection does exist.
2. No seats are guaranteed during strikes, not even if you are in the correct train in the correct wagon. Priority passengers (people with children, disabilities, old people) are preferred to get a seat.

One has to remember that in France, it seems that once something doesn't work exactly as expected, no rules exist. In the train I was told to get off at Avignon and change. It should have been 50 minutes waiting, it was 3 hours waiting. It seems that it would have been better to get as close to the final destination as possible since most TER lines operate (in limited amount, but operate). (Needed to say, a TGV ticket is not necesserily valid for a TER train.)

Officially, no, you cannot take a non-idTGV train with an idTGV ticket, even on a strike day.

The official answer about what happens if your idTGV train is canceled due to the strike is

Nothing! idTGV handles everything!

You will automatically receive an email with your compensation in the form of two vouchers for 100% and 20% of the price of your canceled ticket.

The SNCF website reports other possibilities that I can't find on idTGV's website:

• You can request a refund on your credit card (using the refund request form on idTGV's website, I think). You don't get the extra 20% in that case.
• You can take another idTGV train on the same day or on the next day. Show your ticket to the conductor as soon as possible. Obviously you cannot be guaranteed a seat.

So I'm afraid you'll either have to travel tomorrow or get a refund and buy a more expensive last-minute SNCF train (if there's still room) or find some other way.

An older SNCF FAQ (found by Vince) states different rules — you can take a non-idTGV train with an idTGV ticket, like other SNCF ticket types; but you can't use the ticket the day before with any non-echangeable ticket type (which is possible now with non-idTGV tickets). In the end, what matters is what conductors have been instructed to do — or rather what rules they apply in practice, which isn't always the same. Nonetheless, if you take a non-idTGV train with your idTGV ticket, I'd say the risk of racking up a fine is pretty high if your ticket is checked.