If you bought the game here on GOG, you can start the first game without any additional patch as the basic one is already included. For a replay I recommend to download the whole patch including plus from ModDB or use the GOG plus version to see what wasn't finished.

Can the patch be upgraded mid-game?

No.

Does the Gog Unofficial Patch "Mod" include the Plus version? I presume that when you install it via GOG Galaxy, it's just the basic Unofficial Patch?

Is there a place to check (in game or within the install) which version is installed of the UP (Basic or Plus), and can I upgrade to the Plus version with the GOG UP "Mod", or do I need to go to ModDB to get the Plus version?

The normal GOG version of Bloodlines only contains the legacy option and the basic patch. You can download the plus patch seperately from there though, they just call it the Unofficial Patch mod!

Thank you so much for your reply wesp5 - and for all of your amazing work!

To be clear, I grabbed the Unofficial Patch from ModDB, and checked in the installer, the "Plus" option. In game under options, I only have a listing for UP 11.5. Is there a way to verify that I correctly installed the Plus version of the Patch, and am not just running the Basic UP?

If you have flames and blood drops on the menu screen and Bloodheal as a discipline, the plus patch is active.

///We have both flames and blood drops on the menu screen - game on!

Great news !
Thanks again , wesp5 ! You are still around...
I just finished Bloodlines 2 and... well.. not the same quality compare to Vampire bloodline 1 !