I think Answer1 is the best solution to go with, but if you don't want to use login scripts, then there is a way.
You can import the roaming profile's NTUSER.DAT file into REGEDT32 and modify the drive mapping paraters. You should be comfortable with working with the Registry to do this!
First read Technet article Q146050:
http://support.microsoft.com/search/preview.aspx?scid=kb;en-us;Q146050
(please remove any spaces)
This article tells you the basics on how to load a remote NTUSER.DAT file (which contains the HKEY_USERS Registry hive, where drive mappings are stored). Do steps 1-6 in the article; you want to load the NTUSER.DAT file for one of your remote profiles.
This takes you to step 7. Now, the key you want is HKEY_USERS\S-####(the SID for the user profile you want to change)\Software\Microsoft\Windows\CurrentVersion\Explorer\Map Net
You are gonna have values of a, b, etc., and each one will have a Value Data of a drive mapping.
Just double-click the drive mapping that you want to change; change it to whatever you want and click OK.
At this point, you would go back to the Technet article and Unload the Hive (step 8).
That is it.
The roaming profile should now be changed. The next time the user logs in with the profile, it should show the new drive mapping.