I have had a couple of cels like that... the most important thing to
keep in mind is that, while you don't want to damage a beautiful sketch,
rice paper has no value.  That being said, ignore whatever damage you
are doing to the paper and prtect the cel.  I usually try to simply
lift it slowly and carefully away from the cel, but if that doesnt work,
don't bother using anything special, just carefully rip the rice paper
away from the unpainted areas of the cel.  Unless it is important for
some reason to have the BACK of the cel look nice, it will look perfect 
displayed even if there are a few bits of rice paper stubbornly clinging
to the backside of the painted parts.

Tifa