My experience with full-flex business tickets on international high-speed trains to and from France with mandatory seat reservation (especially Thalys) suggest that
Changing a ticket is in fact quite easy on trains with mandatory seat reservation. In particular, you don't need to cancel or buy anything, you do not even pay a few euros for a new seat reservation, you simply change your existing ticket.
Simply showing up is highly discouraged. If push came to shove, and you went to the platform and started crying, I don't know whether they could or would help you, but in principle you are supposed to check for availability first, even if only a few minutes beforehand.
You cannot use the SNCF app, machines, or counters with tickets bought abroad. I once got turned down at the Gare du Nord because my ticket was booked on NS ticket stock.
On the other hand, my tickets have a customer service telephone number printed on them that has always been very helpful. I used it several times to switch trains or rebook a trip without any problem. If I have a complicated request, they even call me back to follow up.
In fact, it has always worked more-or-less the same way for domestic TGV. Whereas it's not possible to change a discount ticket on a regular train (without mandatory seat reservation), you can easily change a flex TGV ticket. Those two things are simply disconnected. But they want you to change your ticket and not show up unannounced, even if that change is free. They even (used to?) have special machines to beat the queues if you needed to do that shortly before departure. Nowadays, you can do it online of course but I assume you cannot use the SNCF app with your ticket (maybe some DB app?).
So my advice, unless and until you get a better answer, is to look for some DB customer service phone number and try to call them to change your reservation. That has always worked seamlessly for me with NS. If you do it some time in advance, they can email updated tickets but if you call from a taxi on the way to the station, you simply write down your new carriage and seat number and show up at the train with your old printed tickets, no problem at all.
On the other hand, most ICE within Germany work differently and I don't know whether all of these rules apply to the train you're interested in. Differences would not surprise me as, unlike Eurostar, Thalys or Lyria, which are all joint-ventures with a common sales strategy, TGV and ICE between France and Germany are operated completely separately with only minimal cooperation between the two companies.