actually,... studios frequently "repeg" (ie they 
cut off the registration holes and then retape 
registration holes on).

some of the last unicorn cels I've seen have 
had that done to them, and I seriously dont 
think it was a collector who did it.

In fact,... when I worked on Futurama, we 
would frequently repeg the animation 
drawings we were working on, to get a better 
angle, or to change the action of a scene.

though I'm not sure of the exact benefit of 
repegging a CEL (as opposed to the 
animation drawing,...) I figure its about the 
same reason.