1.) First make an iCal backup: Click on each calendar on the left hand side of iCal 1 at a time highlighting it's name and then going to File Export > Export and saving the resulting calendar file to a logical location for safekeeping.
2.) Go to Apple Menu > System Preferences > MobileMe > Sync -- Uncheck calendars so they do not sync. Calendars no longer sync with the new MobileMe calendar.
3.) Go into iCal > Preferences > Accounts and delete the MobileMe account from here.
4.) Remove the following to the trash and restart your computer:
Home > Library > Caches > com.apple.ical and/or just ical
Home > Library > Calendars > Calendar Cache, Cache, Cache 1, 2, 3, etc. (Do not remove Sync Cache or Theme Cache)
Home > Library > Preferences > com.apple.ical (There may be more than one of these. Remove them all.)
NOTE: Removing these files will remove any shared (CalDAV) calendars you may have access to. You will have to re-add those calendars to iCal > Preferences > Accounts.
5.) Once the computer is back up and running go to iCal > Preferences > Accounts, and add the MobileMe account using your MobileMe @me.com email address.
Hope that helps!