Good plastic cements work by melting and fusing parts together. You can't really debond that. But You can still get kits apart pretty well. Someone had blobbed old tube glue all over my Aurora 2001 Moon Bus kit some 40 years ago, pulled the parts back apart and stuck it back in its box. The glue was so old and it hadn't effected the plastic and you could just flake it off. Freezing kits works to make the joints pop apart better. You could try soaking the kit in a plastic safe solvent like Super Clean. That will also remove old paint. And, depending on the kit, you may not need to take every single piece off. If its a figure and the parts are farily well secured, just leave it be, work over the seams, etc. and repaint. For something like an airplane maybe work on some of the smaller parts, so fixing up the big assembly is easier wihtout the risk of breaking small parts off.