I agree. I believe that a game should always offer something that you can't get anywhere else. Or at very least offer it in a unique and original way. So if you're tired of seeing the monster sets and believe them to be over used than imagine what the players must feel like. If you feel your project will benefit from these tweaks than go for it. If you're still torn, than make a back up of your project before changing the sprites. If it feels weird or wrong then just restore the previous version and be done with it. To be honest a lot of the best things in my project I stumbled across by accident or just by trying several things.