How to change a Deutsche Bahn flex ticket with mandatory reservation

This document might answer part of your questions. Starting on page 69 of the PDF document, you will find this:

2.4.3 Fahrkarten, Reservierung

Reisende mit einer Fahrkarte „Flexpreis“ können die zugehörige Reservierung für den gebuchten Reisetag im personalbedienten Verkauf kostenfrei umbuchen, sofern Sitzplatzkapazitäten im gewünschten Zug am selben Tag verfügbar sind.

Passengers with a flex ticket can change the reserveration free of charge at a staffed ticket office if seats are available for the desired train. It's not clear though whether this is possible at an SNCF station.

2.4.9 Regulierung im Zug

Reisende mit Fahrkarten und Reservierung für einen anderen Zug am selben Tag , zahlen im Zug eine Fahrkarte zum Angebot „Pass 1“ plus Bordentgelt, wenn es sich bei der ursprünglich gebuchten Fahrkarte um einen „Flexpreis“ (...) handelt.

To my understanding, this implies you can simply board a different train on the same day and have your reservation changed by onboard staff for an additional fee. The fee is "Pass 1" (13 euros 2nd class, 20 euros 1st class) plus the onboard surcharge of 15 euros. From experience, it's always better to actively approach a member of staff than wait for someone to turn up.

2.4.10 Umtausch, Erstattung

Eine Fahrkarte zum „Flexpreis“ kann bis einen Tag vor dem 1. Geltungstag kostenfrei umgetauscht und erstattet werden. Danach sind Umtausch und Erstattung gegen Zahlung des Bearbeitungsentgeltes in Höhe von 19.00 € zugelassen, wobei der Umtausch nur im Rahmen der Verfügbarkeit möglich ist.

Finally, you can return your flex ticket free of charge before the first day of validity. If you return the ticket later, DB will keep a processing fee of 19 euros. Online tickets can be returned by filling in an online form, or sending in a printed form by mail. Tickets bought at a station can only be returned at a DB counter.

My experience with full-flex business tickets on international high-speed trains to and from France with mandatory seat reservation (especially Thalys) suggest that

1. 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.

2. 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.

3. 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.