I know we can take NPCs into CAS using MCCC or cheats, but do the changes stick? Is it safe to edit them in this way?

In my experience, NPCs altered by MCCC are indeed somewhat different than game created changes. But yes, the changes stick, and can be edited. But now, since this new update, I don't know. I'm testing, like I suppose everyone else is.

Thank you! Coming from TS2 I'm always paranoid about messing with NPCs :p

Editing them in Cas is no problem. Their work uniforms are issued via a different mechanic so there's nothing you can do to them in Cas that would mess that up.

Most of these NPCs are issued with a unique hidden career and a unique hidden trait that ties them to that career. Moving them into a residential to play them normally will break that for most NPC workers, though not all. The hidden trait and career is coded to be removed, meaning they will be replaced with a new NPC sim to fill the job.

The jobs are coded for the NPC sim to be on call 24/7, so they would be a pain to play normally as they will always be sad for missing work and if you dared send them to work they will never come home again due to 24/7 work shift. Hence hidden career and hidden trait needs to be removed if trying to play the sim normally.

The worst one of the bunch is the NPC Hermit from the camping GP, such that all that sim needs to do to lose their hidden career and hidden trait is to turn up on any lot. If you do see them randomly visiting a community lot, know the next time you go to the hermit lot it will be a new NPC there to replace them. Mods can fix this.

Even so, you can still take the hermit into Cas and make changes without any problems of them losing their job.

I've never had any issues editing NPCs in CAS, their changes usually do stick. I've never tried changing non-physical stuff about them though, like their name or traits, so I'm not sure if it'll break events and stuff like that if you edit someone like Emit Relevart in that way.

Father Winter is another NPC that sometimes just show up, or being one of the "caller Sims" that tells you to get married or get showered in gifts for a "favor". At times I've found the original Fanter Winter has been moved into a lot by MCC and replaced with some random townie. It's a little immersion breaking.

That bit about hidden traits and 24/7 shifts makes so much sense now. I’ve definitely had NPCs “disappear” after inviting them over and wondered what broke. Do you happen to know if editing an NPC’s outfits only (like just their cold weather or formal) also risks breaking their assignment — or is it safe as long as I don’t move them into a household?

Yeah, taking them into Cas - as homeless - just for that is safe. Also a concern of recent times, if using a mod to gain control of any sim, it may well break the working assignment, the hidden trait, of some of the more stricter NPC jobs. Not all, as not all have such a strict condition in their hidden assignment trait. I'd bet on the Hermit from Outdoor Retreat would lose their role and be replaced if you took control of them. Sheesh, they lose their role if they so much as turn up as a random visitor on any community lot, never mind directly controlling them. Definitely needs mods to safeguard them always losing their roles at the drop of a hat.