How do you know when a page needs a rewrite vs just an update?

One thing I'd add — check your bounce rate and time on page before deciding. I've had pages that ranked ok but users were leaving in like 10 seconds. No amount of fresh data fixes that, its a structural problem so thats when you rewrite. If people are actualy reading it and just not converting or ranking well, an update is usually enough.

Just check GSC and look at the traffic trends for the page. If clicks are dropping or hit zero, the content is probably outdated.
Sometimes you don't even need the data - you just know things have changed and the info doesn't hold up anymore.

That's the neat part, you don't. Well, you do, sort of. Basically, if I’d keep most of the existing content, it’s an update. If I find myself questioning the structure and what the page is actually trying to accomplish, that's a rewrite. Good rule of thumb.