Where should I be looking/learning about the changes between Ubuntu (server) editions?

I maintain a few Ubuntu servers and it works great... but, I now have notifications on a few that state there is a new release available and I should run do-release-upgrade.

Since these servers were initially provisioned, I have create a few extra ones that are on the newest version, and, I have noticed quite a few differences.

For example, perhaps I was doing it incorrectly, but, I used to set DNS servers manually in /etc/resolv.conf, but, on the latest version, that file is created/modified automatically and overwritten (and I am still not 100% sure where from).

(Skip to here if you don't want the back story!)

Before I deploy a new version, I really need to know almost every difference and make sure it doesn't break anything... Is there any (official) place to look at that can tell me critical or important differences between the versions?

Are the release notes the only place, or, are there better guides anywhere?

Checking the release notes would be the recommended solution. One thing, they are thorough and covers most of the major changes between default software packages in the server. Here's the release notes for 12.04.1 server.

If you want to get even more granular details of the changes, pour through the changelogs of a particular package.

Actually Ubuntu server is the same as Ubuntu Desktop with some modifications in built-in packages preinstalled and some configuration for server tools

check this link and this other link on what's new for ubuntu Server anyway.