http://www.modthesims.info/download.php?t=89766&c=1This stops sims and NPCs changing back into their own clothes. I use it in conjunction with the Boutique and don't think I have any issues, I have all EPS up to M&G-although just because it works ok for me doesn't guarantee the same for you. You need to register at Mod the Sims to download.
edit: some NPCs cannot be changed with the Boutique, the social worker for instance, so the NPC Replacer needs to be used for that. The Boutique will also not allow the food judges or the break dancers to be changed although the option is there for them, when they turn up they just stand and do nothing. The NPC Replacer only covers NPCs up to NL I think so cannot be used for those NPCs. I suspect SimPE may be needed for those sims.