You'd have to buy a new saddle. I've never heard of altering a saddle, and if it's possible I imagine it'd be cheaper just to buy one.
If he's not in pain, he's being a brat. I've heard of three different ways to help a horse that is cinchy. You may want to give him a treat while you put the saddle on. For some reason, this solves some horses problem because they start seeing saddling as a good thing. Of course this may encourage bad behavior or make him nippy. Another tactic I've heard is tacking them up in an area where you can lunge them if they start acting up. I personally just smack them when they start acting stupid, but depending on the horse the other two methods may work better.
What kind of girth do you use? Shannon lost a lot of her cinchiness when I started using a more comfortable girth.