Removing to Reapply Decals
I usually have the exact opposite problem, I can never keep mine from cracking and peeling.

If the model wasn't sealed, you could have tried pressing some masking tape over the decal and peeling it off.  That may take off the decal with out taking off the paint underneath. 

Once the model is sealed, I don't think there's a good way to get the decal off without taking the paint with it.  Any kind of chemical you use will need to get through the sealer and would probably dissolve the paint as well. You'll probably have to paint over the area and apply the decal on top of the old one.