If you didn't find it in the UI or in the manual, it's the fifth item in the FAQs list here:
I think if you've customized your toolbars a lot then it shouldn't be hard to find the old toolbars after an upgrade, since you'd probably be familiar with how Customize works. And if you haven't, then you don't usually need to worry about the toolbars.
We do plan to make a future installer which automatically backs up settings for you. That is a good idea for lots of reasons.