From the command line:
p4 -c wkspc1 --field Root=C:\p4wkspcs\wkspc1 client -o | client -i
p4 -c wkspc2 --field Root=C:\p4wkspcs\wkspc2 client -o | client -i
ren C:\Perforce_windows_workspaces C:\p4wkspcs
This handles the actual renaming -- if you had a large number of workspaces this is obviously pretty easy to script. If you're using an old client and --field doesn't work, you can do the first two commands as just:
p4 -c wkspc1 client
p4 -c wkspc2 client
and edit the "Root:" field manually.
To double-check you didn't make a typo and that the rename worked right, do:
p4 -c wkspc1 diff -sd
p4 -c wkspc2 diff -sd
These should both return no output (they're checking to see if any files you've previously synced are no longer present in their expected locations).
From P4V, you will need to switch to each workspace individually and edit the workspace to change the Root field. You can't rename the folder itself from P4V; open an Explorer window and do the rename there. You can use "Reconcile" or "Folder Diff" to verify correctness once everything is done.