> I understand all the tags and captions I embedded into the images with Picasa will be recognized/utilized by Thumbsplus.
If Picasa uses standard EXIF/IPTC fields to store them, yes.
> How can I upgrade? Do I have to find my old program and reinstall before upgrading?
No, just purchase an upgrade code, download and install the software and activate it using the code.
> Can I upgrade from my old single user to the Professional or Professional Perpetual?
I think you can, but please ask Cerious directly by email (put 'thumbsplus' in the subject line). This is a user-to-user forum and Cerious' staff doesn't visit here all that often.