I too have a sparpreis Brussels-Frankfurt-Kassel Wilhelmshöhe and return as far as Frankfurt Airport next month. I've had two separate emails saying my itinerary is no longer possible and my ticket is now flexible, both ways.
The emails aren't very helpful. Outbound suggests an alternative on a regional train, but on researching, my ICE from Frankfurt is still running at the same time but with a one hour extended journey time. The suggested alternative on the RE direct to Kassel actually took longer!
Returning, again my ICE is running but retimed an hour earlier at 6am. No alternative was offered in the email. I need to board a flight from FRA at 11:15.
Actually I was wondering if I could travel on it the night before as an alternative?