1) Keep the old one as a stand by until you know the new one works.
2) Drivers, drivers, drivers.......... are they available for all your hardware on the new OS ?
3) Personally speaking, all my projects / back ups reside on external drives, so no transfer necessary.
4) When you hit that point where you wish you'd never started, keep going. It
will (probably) be worth it eventually.
5) It's ok to swear a lot when trying to re-authorise all them plugins is nowhere like as straightforward as it should be.
:)