Well, I managed a work-around, although I would still like to know the orthodox way to do this.
What I did was:
(1) (Perhaps unnecessary) Since VM was encrypted, checked "Remember this password" when I logged on.
(2) Copy the entire (80Gb!) VM's .pvm file to another hard drive as a backup.
(3) Then, on my primary hard drive, manually renamed the VM's .pvm file from Win7-Parallels.pvm to Win10-Parallels.pvm.
(4) I was able to open the renamed *.pvm file, which showed up in the Parallels control center. Once it was opened, but before I started the VM, I was able to use Parallels' configuration options (under the cogwheel of the Parallels control center) to change the VM's display name to correspond to the filename (i.e., Windows 10 instead of Windows 7).
As I say, I'd like to know a cleaner way to do this, if there is one.
Last edited: Sep 23, 2015