Yep!
It might be worth using Opus 8 to back up your Opus 8 configuration first, just in case.
(I think the installer will offer to backup the config for you as part of the upgrade but I can't remember for sure. Only takes a second to do so you might as well.)
Yes. The previous Opus 8 licence won't be recognised by Opus 9 and it should prompt you for an Opus 9 licence shortly after you install. (If it doesn't, you can use Help -> Licence Manager.)
Yes, it is possible. The installer convert your Opus 8 config to an Opus 9 one. (I can't remember if it just does it, or if it asks whether or not you want to do it, but it's definitely possible.)
Whether it's worthwhile depends how much you've re-configured things.
If you've put a lot of time into your config and would miss it then you'll probably want to bring it to Opus 9.
On the other hand, if your config is largely the default that came with Opus 8, then you'll probably benefit more by not bringing it over. Instead go with the default Opus 9 config and change the few things that you changed before.
The default Opus 9 config has a bunch of extra things (toolbars/menus/hotkeys) added to it which you might find useful, and which you won't get if you import your old Opus 8 config. Of course, there's nothing that you cannot add/configure yourself. So it really depends how much you are into customizing Opus what the right answer is.