Every version change, even minor revisions, come with a real risk of breaking your compiled asset bundles. From shaders to light maps to referenced classes. It's a huge amount of effort to keep a project tracking Unity version upgrades.

What do people do to deal with it? Are there services / libs that help you do the conversion?

Or is it that you can't fully trust them, and you'll need to do that by hand anyway, because you might be one of the exceptional cases?

From my limited experience, download the version that it was made with. Unity has a list of older versions right in it's hub.

What's funny is I think all of the major game studios that I know of essentially lock game engine per project once they're coding along. It's just expected that you stick with it for the rest of the project. Maybe 1 upgrade mid project. I had heard bungie used to, in the halo days, lock the engine 8-11 months out and the core tech team started working on the next game. The AAA studio I worked for essentially did that 3-6 months out.

But those were 2-3 year game cycles. Maintaining a project would be less fun. But it'd be like choosing to do a rails or react upgrade.

Yikes. Does Unity do forced paid upgrades, so you'd have to deal with the incompatibilities?

Do they have a culture of incompatibilities so they can simplify their development and keep the best features and discard the worst?

Unity is both free and subscription based.

They make older versions of the engine available in their management app, and projects are linked to a specific version unless you explicitly upgrade.