If you had a save game that was in a different zone, you could get around the problem. The game saves the very last zone that you were in (in the New.Lod if I remember correctly), so as long as you are reloading the same zone, the changes that you make will never be picked up. If you load a game in a different zone, and then go back and reload your game in the zone you changed, any changes should be picked up.
By the way, there are two references for each monster type and level in the mapstats.txt. One of them refers to the monster definition in monsters.txt and the other one defines which picture to use for the monster. You can change the monsters stats in monsters.txt, but changing mapstats is only changing the picture.
