TGV question

Are you exchanging your ticket Amboise to Rennes to leave two days earlier?
If you are leaving the same day from Paris to Rennes as planned then your ticket is valid.
Did you get an a separate e-ticket each with a QR code for each leg Amboise to Paris and Paris to Rennes?

Thanks for your help. From what I could tell from reading the print on my e-ticket, I'll have to wait until I get to a train station to exchange or get a refund so I went ahead and bought new tickets to Paris a few days earlier. Then I want to use my old ticket that would have started from Amboise to Montparnasse to Rennes and just get on the train to Rennes from Montparnasse. The e-ticket from Amboise to Montparnasse to Rennes is all together on one QR code.

You'd be fine. Those are your seats no matter where you board. You may have to boot someone out of them, but they're yours.

I'll add some nuance to Bets' comment for other travelers in similar situations.

In your case, you will be fine, since your original ticket is for 2 trains (Amboise to Paris and Paris to Rennes). The ticket gates in Paris will not magically "check" if you were on the 1st train from Amboise to Paris.

However, in a situation where you have a ticket from A to B on the same train, and want to board in middle station "C" instead, you will not be able to do so if there are platform gates in station C. Only major stations have those gates, but still, I learned this the hard way trying to board a Marseille-Paris train at Aix TGV instead of Marseille. I was able to argue my way in with the agent, but it was not pleasant.
If you want to get off the train at station C, that's never an issue.

Thank you all for clarifying how the train system works. I'm much relieved and glad that I won't have to do a lot of explaining on the way to Rennes.