I would strongly urge people NOT to do anything like that. It is very easy to get tangled in knots with the various versions. There are lot stacks that require Stacks 3 -- once updated the content built with those can't be easily migrated back -- and finding older versions can be non-trivial.
In my opinion, the best way is to move forward -- but do it when the time is right for you.
That said, the one solution I have found that is rather foolproof in this regard is to keep a separate user account for the older version -- as the developer I have no choice but to switch back and forth continually -- and this is what I do. This will ensure isolation of not only the Addons folder, but also the files that you open with it.
Short of that, I think you're really tempting fate -- and I'd just urge a really solid hourly Time Machine backup in case you need to jump back.