Say we have a character that has multiple s and makes a ranged combat attack targeting multiple characters. Say that one of the targets makes a successful Shape Change roll. Would the attacker have to commit to only attacking the character(s) that he chose in the first place (minus the Shape Changer), or would he be able to reselect his targets?

The former. Only if all targets of the attack become invalid does he get to redeclare.

Ok, that's how I thought it worked. It came up at a tournament today and nobody was sure of the correct answer.