It is already past midnight and so I do not want to write a long post.
As I perceive, your query to clear all current drive letter assignments has a deeper meaning than just reassigning the drive letters.
I shall redirect you to have a look at this:
USB drive or flash problems? How to cleanup and remove old USB storage drivers - TechSpot Forums
OK. Download and run v 0.8.1 of the drivecleanup.zip. And if you so wish, you can right click in the appropriate .exe file and run as administrator, instead of dropping it into the System32 folder. After the clean up act, reboot and plug in your USB devices in the order you want the drive letters to be assigned starting from the first available one. And you will be starting with a clean slate.
( I use it almost every month to clear the cobweb.