Okay, I'm officially FrankenPre'd! I haven't received my practice Pre but got a bug to try it anyway. It was slow going mostly with the reassembly (sliding the bezel back in was difficult as was the one black wire for GPS). I tested the phone and it works. currently the apps are being restored.
I have not yet set up my Doctor to 2.1. That might have to wait a bit unless I get another bug.
I thought I had done a good back up of my Pre before the surgery (copied the contents to my desktop) but it turns out I didn't and some important personal stuff was missing.
I decided to try to see if I could retrieve that information from my old Pre without re-installing the Sprint comm board. Originally, I opted not to put the comm board from the Verizon Pre 2 into my old Sprint Pre, so I just installed the battery and connected the sprint Pre with the USB cable. No go - I saw the battery symbol but it would not connect to my desktop.
Next, I put the Verizon board in the Sprint Pre connected it to the desktop and ... success! It booted up just fine and did not ask me to sign in or register or anything. I was able to go into USB mode and move everything onto my desktop.
I'll be using the Sprint Pre to charge the extra battery I now have, so it won't go to waste.