What happens between Unity versions?

We're not at this stage yet, but it's something we're worried about. Let's say we release our product with Unity Version A, using Addressable assets to keep the initial deployment load down. Then a few months later, we start using Unity Version B and update one (or more) of our Addressable packages, publishing this to our own Content Delivery System (for reasons). Would the end user still be able to seamlessly load this Addressable 'thing' when they are running the launcher built with Unity Version A, or would this fail?

Inversely, what happens if we update the main deliverable launcher to Unity Version B but somehow miss updating a Addressable package that is still using Unity Version A. Does Unity check which version the Addressable file was made with and refuse to load, or does it try anyways and does (mumble thing)?

I read the docs and blogs and etc but didn't find anything that answered my question. If there is an official answer/doc on this, a pointer to it would be super helpful.

 

I've gone through a few simple use cases with Addressables and upgrading Addressables using remote catalogs / bundles on Android/IOS. I haven't sat down to scientifically test it, so I can't answer all your questions, but some gotchas I know of:
-Building with a new Unity version will always change the hashes of all the bundles which will trigger a rebuild all bundles by default. (there appears to be a setting in Addressables for "Player Version Override" but I'm not sure if that will change this behavior)
-If you have local bundles (i.e. the ones deployed with your client via StreamingAssets), ensure that local bundles are set to Bundle Naming Mode "File Name", as the hash of all the bundles will change in the new catalog and will cause the old client to not be able to load it's own packed assets anymore, as it will complain that that local files have the wrong hash.