The very first thing you need to figure out is
Do I need to upgrade at all?
Upgrading is a change. It's a massive change, actually. Is the cost of testing this change worth the benefits? What are the benefits? Do you have a backout plan if something breaks?
Personally, I would upgrade on a case-by-case basis. If version 2.44 of the Floopy module works for you, why does it matter if the latest version is 4.55? If you never hit a bug, then you don't need the bugfixes. Plus, bugfixes might introduce bugs you're not aware of. If you need a feature in 4.55, then upgrade to it.
Furthermore, the issue isn't bugs within a given module - it's bugs between modules. Integration testing is expensive! And, testing doesn't prove the abscence of bugs - it simply shows you haven't found any yet.
Thinks for a second . . .
The biggest problem I have with your plan is that you're changing too many things at one time. Change one item, then see what happens. If nothing breaks, bake it in for a month. Then, change another thing and see what happens. Lather, rinse, repeat.
Being right, does not endow the right to be rude; politeness costs nothing.