The apps included in the Creative Cloud Membership are installed individually, so you could just not install FB and leave the current stand alone install you have as is.
Just two questions
1. Is the FB 4.6 version in CC the same as the FB 4.6 I installed in December, or is it a newer release (eg 4.6.2 etc)? If the CC version is the latest would want that
2. If I did keep my current FB4.6 -- would the CC "know" that it should receive the latest updates. I keep reading where CC apps are always up-to-date with latest releases and builds -- so if I have an "old" FB4.6 -- would the CC updater give it the same updates as if I had installed it directly from CC?
I think its the same version as you installed previously. I'm just seeing 4.6 as the version number.
For switching it to subscription, it is possible to do so without reinstalling. You would just need to deactivate it from the Help menu, close and relaunch it, after which it will be in trial mode and you can sign in with the Adobe ID associated with your subscription (when active) and use the "License this product" option.