What do i need to upgrade a Mac?
I'm not sure what you mean. You need to be more specific. If it's that the Mac is tied to an Apple ID, no: only the current installation and user accounts are.
Yes, you'll need one to download the installer unless you get it some other way (someone else downloads it, you use internet recovery, etc.)
Sort of. To my knowledge, you can only do so if you have a Time Machine backup of relevant files from before your upgrade. You'd be able to move most changes back over once you finished the downgrade, but I don't know of any easy way to restore an older version directly from the current one without losing data.