'Morning--
Changing your ID really wouldn't accomplish anything (I don't think) because everything else stays with you, FB, email, password, all of it.
If you click on anyone's FB profile, to the right you'll see links for contact member, items for sale, and more options. If you click on more options it gives user ID history, here's an example I'm going to use (with permission from this member)
User ID Effective Date End Date
sparxx001 Mar-07-07 Present
crazymouse3 Nov-28-06 Mar-07-07
sparxx001 Oct-17-06 Nov-28-06
To save yourself some aggravation, you should probably leave it alone...just my 2 cents