Agreed, Impellers are one of those change-every-year maintenance items anyway, so pull the old one however you can, and throw it away.
Perhaps the best bet for easing the old one out is to spray some soap solution into the pump/impeller, and bump the engine over a little bit to distribute the soap (and get it between the impeller/housing). Use the paint can openers, or vise-grips, and pull it out. Wait till spring to install the new one.
1998 Maristar 200VRS