Are you not working with a partner right now who can answer those questions for you? If not, we'd be happy to help.
You can upgrade for sure, and it would be pro-rated. I don't believe you can 'downgrade' and get a credit for what you've purchased though. It might be possible but 3CX CS would have to get involved. But otherwise you'd buy a new 16 license and then swap keys in your instance to keep all your settings/FQDN since you are annual anyways