If they are close to the surface, try using a dremel (or equivalent) with a small grinding wheel to cut a slot in the post and then use a fine screwdriver to remove.  If the playfield gets scored, it is quite clean (easy to repair) and should be covered by the new post.  An alternative for stubborn screws is to drill a series of fine holes around the screw, then use a pair of fine needle nose pliers to try and screw the post out.  Again, most the damage should be covered by the new post.

cheers
Ian