Question: I have a reservation for a TGV train that I won't be able to make due to major delays on the DeutscheBahn. I was given a new suggested itinerary by a gate agent, but she did not make an actual change to my ticket or reservations. It involved being on the subsequent TGV.

The TGV as I understand it requires reservations, so I am wondering if this will cause a problem? I also don't want to end up not having a space on the train or a seat as I have two young kids and an airplane to catch from Paris tomorrow. 

If it will cause a problem, how can I get a reservation for a seat? The DB agent said she could not arrange it but told me it would not be a problem, but I don't know that I believe her. Unfortunately I have very short changeover times from here on out.

Answer: 
What ultimately ended up happening was just as the DB agent said: I made my train (with one *more* delay, as my RE engine overheated and started smoking...) and had no trouble going the rest of the way.

It worked out in part because the train was originating in a different location (originally was supposed to originate in Karlsruhe, instead originated in Mannheim) and so was effectively a new train that nobody could buy tickets/reservations for. We ended up having the entire bottom of a first class car to ourselves and never were even asked for our tickets (I think I ended up going the entire route, 4 trains, with only being asked for my ticket once).
Don't worry, whenever in France you cannot catch up with your TGV or TER, use SNCF App on iPhone or other phones, find when is next TGV/TER to your destination, get on it, find a seat, and rest until conductors come to you. They know all about delayed/cancelled trains and will not bother you at all but will even help.