Reverting to old Unity version after upgrade

Hey!

I want to upgrade my Unity project to the newest LTS but it is a huge project for a client and I can't risk any major problems.

So, if I upgrade to a new Unity version and there are problems, can I just revert to an old changeset and it is treated as the previous Unity Version? Would I have to do this outside the Unity environment? I can't imagine how it will solve this from inside the editor of the new version so I'm asking here before doing anything.

Thank you!