Rightly as Guillaume said, tampering with a system table isn't a very good idea.
This is what he suggested:
1. Lets say you are connected to Database A
2. Reconnect to Database B on the server (this database B could be any database other than A on the server)
3. Rename database A to C
4. Reconnect to C
Without step 2, step 3 would always spew the error about unable to rename the 'current database'.
Robins