I want to change some of my townies' careers. I figured I could have them over, make them selectable, and cheat with the newspaper to give them a new job, but is this safe to do? Has anyone tried it? I've been searching but I can't find anyone who has mentioned it.

Or is there another way to change their jobs that is definitely safe?

Yes, this will be fine. I've changed them in SimPE but I don't know of any way to do it that isn't safe, as long as you leave the dangerous NPCs alone.

Remember that the chief reasons for corruption are missing data/extra data hanging around in the code. Normal game functions like changing jobs don't leave unattached data behind them when performed.

Rule of thumb: If a townie or NPC hasn't got a proper name (Unsavory Charlatan, Hot Dog Chef, Social Worker) and/or cannot be interacted with normally without cheats (Mr. Humble, restaurant chefs), they should be left alone. Otherwise, they're just like playables.

Townies are not the same as NPC's. If you grab a passing townie like Goopy you will see they have a rabbit hole job under the employment panel that a sim can get from the newspaper such as dishwasher or captain hero, you can change that to whatever you want in game on the simblender. NPC's are sims you see doing a set job. These include hair dressers (don't make hair dressers selectable) the teen behind the till, the mail carrier, the firefighter that shows up at your sims house, the nanny etc. Do not change those sims jobs. Now most of those sims you can move in eg the nanny. Once moved in they stop being an NPC and are now a playable so at that point yes you can change their jobs.

Thank you both so much!

character types game generates jobs for; I take safe to change jobs of such characters.
character types that have special coding; probably better to make them permanently playable before giving a job. if the character is a safe type.