Consider the following:
We have an account named MYDOMAIN\eholz. This accounts Active Directory Login Name changes to MYDOMAIN\eburrell
Now this user was a active user in a Sharepoint 2010 team Site, and had a userProfile using the Account name MYDOMAIN\eholz.
Since the AD LoginName changed to eburrell hence we need to update the Sharepoint User (SPUser object) as well update the userprofile to reflect the new account name.
To update the Sharepoint User LoginName we can run the following stsadm command on the Server:

STSADM –o migrateuser –oldlogin MYDOMAIN\eholz –newlogin MYDOMAIN\eburrell –ignoresidhistory

However to update the Sharepoint 2010 UserProfile, i first tried running a Incremental/Full Synchronization using the User Profile Synchronization service… this did not work. To enable me to update the AccountName field (which is a read only field) of the UserProfile, I had to first delete the User Profile for MYDOMAIN\eholz and then run a FULL Synchronization using the User Profile Synchronization service which synchronizes the Sharepoint User Profiles with the AD profiles.

Update: if you just run the STSADM –o migrateuser command… the profile also gets updated automatically. so all you need is to run the stsadm –o migrate user command and you dont need to delete and recreate the User Profile