I've got an older version of UBB, 5.45c. Can I still upgrade and keep all my settings and members?

Yes, you'll first need to upgrade to UBB.classic 6.7.3. At that point you can install UBB.threads and run an importer to import all of the data. Everything you need is in the Members Area.

If you'd like someone to do the upgrade for you, we have upgrade services available.

You'll need to make sure your server has PHP 4.3+ and MySQL 4 or 5.

You will need to upgrade to the newest version of classic first due to the different file format from the ver 5 series to ver 6. Then you can upgrade to Threads if you wish.
Of course you will need member access for the upgrade but you will have access to both when it is done.
I feel your pain because I waited quite a while myself to upgrade.