I recently did the TGV from CDG to Lyon as part of my AA flight. You will be given a paper ticket, as you need to redeem the flight coupon for the TGV segment at the ticket office in the CDG train station. There is a separate line (usually no other passengers are in it,) to the far right, with the sign for air/rail customers. They have a train ticket they will issue to you at that time. It's for a second class pre assigned seat.

I don't know if you can exchange this, though I know it is marked that you may not exchange or make changes. In fact, you can not even get your return ticket if you are going back to CDG on the TGV from Lyon, until the morning of your departure. That is collected in Lyon using the same process, at the ticket office in Lyon, again the air/rail line is to the far right.

So, it is unlikely that you would be able to accomplish this. But then, it never hurts to ask.