I'm not sure what to do in this situation, but i definitely don't want my dentures to break!
Very Likely Fit to Gums, but Ask Your Dentist
Dentists seem to be kind people. I'm sure they would love you to keep those expensive dentures. Ask your dentist about this. The dental Health Foundation of Ireland specifically suggests not playing contact sports with dentures in. As martial arts are at least as demanding as contact sports, I would think this advice applies to martial arts as well.
However, it seems to be common sense that you should fit your mouthguard to whatever you plan on fighting in. If you want to fight with your dentures in and risk them breaking, fit the mouthguard to your dentures. If you don't want your dentures to break, fit your mouthpiece to your gums.