Such a shame! But not unworkable.

I have cels that were taped to their backgrounds. 
Usually they're only taped at the top, but I've 
seen ones also taped on either the bottom or the 
sides -- or both! o_0;  I've also received cels 
that were taped to their backgrounds previously. 
The damage is usually obvious. (Stickiness on the 
background, tearing of it's surface, or yellow 
marks in the shape of the tape.)

The only way to remove the cel without ripping 
the background is to cut the tape right along the 
edge of the cel, so you're not actually pulling 
the tape off the background. (The tape stuck to 
the background stays put, but the cel is free.)

The only alternative is if the tape has degraded 
to the point of having mostly lost stickiness. 
Then it can sometimes be gently pulled up without 
damage to the background. . . But I, myself, 
never try that unless the tape is almost falling 
off on it's own, and it's always a risky task.

Many Sharp Smiles,
--Drac