It would be difficult, since the torque converter and the transmission input shaft will hold the back end of the crankshaft up tight against the seal. There's enough slop in a manual transmission input shaft to let the crank be lowered a few thousandths without disassembly. If you're dealing with a rope seal, I'd guess no. Changing a lip seal without disconnecting the trans from the engine MIGHT be possible, but difficult.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!