I don't know, but can you change their outfits? Maybe Plan outfit, and is it on all your saved files?
Well, I can change their outfits but if they go to sleep and get up next morning they will wear their formal clothes again, and the same if they go to swim, take a bath or wohoo or whatever. Some of them also wear their formal clothes to work instead of their normal uniform. And I could just keep changing their outfits, but the thing is my computer gets really laggy when I do that and it's annoying...specially because there are seven sims with the issue and not just one or two. Oh, and I don't know about other saved files... I try to play only one at a time because having multiple saved files makes my game slower... but I will check it out tonight and see if it happens.
I noticed in a family recently that the game seemed to be randomly creating alternate everyday outfits for my sims. They didn't use them unless directed, but when I asked them to change clothes all those extra options would come up. I also noticed that whichever I chose to use became #1 and the former #1 got bumped down to #2 or #3. Is it possible that the formal outfits they have also overlap and can be used as everyday outfits, even if you've chosen them only as formal? If so, I'm wondering if the game hasn't randomly made an extra everyday outfit for each sim, identical to the formal one. I know in CAS if an outfit overlaps more than one category, particularly between everyday and formal, the game will suggest the same outfit for both categories. Might be worth checking on. If this is the case, you just need to ask them to change into the desired everyday outfit, making the extra an alternate which makes it delete-able.
I had never heard about this, but I checked if they had multiple everyday outfits and they don't. They just have one for each category and their working outfits. And some of them are wearing multi-category items (I mean, items that are both everyday and formal clothing) but not all of them so if it is overlaping I think it's a bug?
or maybe try Edit in CAS cheat?
Yeah, I tried that... but even if I change their formal outfit, they will put on the new one as if it were their everyday clothing.
I edited a sim in CAS earlier today and changed her everday outfit. At first when she came out she was wearing it, but she wasn't completely loaded yet. Then when she did she was wearing her athletic clothes! She never changes out of it except sleep and swim like you said Frey! 
Really? Well at least I' not alone in this
. Now you say that, the issue might have started with CAS in my case as well. As I said, it started after my sim went to prom, but right before that I edited her formal clothing in CAS, and I did the same thing before the other sim got married and the family attended the wedding. I can't be sure which of both actions started the problem, but I would bet for CAS after reading your answer.
Thanks to all for the replies... I will do some testing, maybe I figure this out during my last days of vacations hehe. Oh and sorry for the long post!
