Go to control pannel, switch to classic view if it is not in it, administrative tools, computer managment, disk managment, click and select a drive or partition, go to actions, all tasks, change drive letters and paths. That will allow you to change the order of your drives. If you select a drive letter that is already used, I believe it will automatically reassign that drive a new leter.
Be carfull though. If you change the drive letters of any exsisting drives, it may cause problems with some software as they may go looking for files in a particular location that is not thier anymore (because technically you moved it when you reasigned the letters).