Is there a way to delete all 'history' of a sim so they're 'new'

I think the best way is to go to cas, create a sim with genetics to make a twin and then dress up the twin the same way as the original so the twin can replace the original.
skills, careers and aspirations save to the gallery, so going to the gallery won't work.

If it is a sim you originally created in CAS, in a new household, the game would have saved a blank copy in your game library. If you haven't deleted that one, that's the easiest way to start fresh.
If the sim was born in game, game generated, or added to an existing household, then this will not be the case. And, as mentioned above, creating a twin in CAS is yours best bet. In my experience CAS twins are never 100% identical. There's always a really small feature that is just slightly different. But you can either adjust that or just go with it.

One thing I know is possible is usage of MC Command Center to copy and paste likeness and outfits to another guy (I don't know about personality traits but if you're using MC Command Center, you can easily change personality traits around with cas.fulledit). There's also the cloning machine where you can clone a guy, move him out to his own household, rename him in CAS, and then save him to gallery but that requires one of your Sims being a scientist to use.

I also know you could manage traits and delete memories from there, forget all your skills and forget careers, relationships do start clean if you start up a new save even with a Sim who has everything done.

Ok thank you for all the tips.

Also, one of the difficulties is forgetting yourself, what that Sim used to be or could do, too. :open_mouth: