Ok, I found an installer for ST 2.55b on the IK Multimedia site in my user area, and proceeded with the uninstall on C: / reinstall on D: procedure described above.
When I ran the ST2.55b installer, I had to manually specify about 6 particular folder paths for VST plugins, RTAS plugins, etc. I just changed C: to D: in each case, and that seemed to work fine.
I had to copy over my saved BB-installed instruments into the new D:\..\ST2.5\Instruments\BB folder. (I had to create the Instruments\BB folders under the ST2.5 folder.) BB printed out a very nice error message telling me exactly where to move the folder -- I was impressed!
On the first boot of BB after all this, the ST plugin asked me to specify my Instrument path, and it picked up the BB-shipped instruments.
So I think I'm good to go on the D: drive.
---------
The only issue I have now is what is displayed in my melody track in my BB song. I had previously assigned a BB-provided "HQ 001 01 Piano, Acoustic, Grand SampleTank.gs" to the Melody track.
But I wanted to use a simple GM assignment, so I did (#106 Banjo). The banjo plays fine in the melody track, but now the melody track text says: ""HQ 001 01 Piano, Acoustic, Grand SampleTank.tgs Instrument: 106" (which is the banjo).
Is there any way to make the "HQ 001 01 Piano..." text go away?