I ran into an issue with the migrateuser stsadm command. When I tried to run it using the Farm Admin account I got a "Value cannot be null" response. I searched for a solution and came across the following article:
http://sharepoint.nauplius.net/2011/06/move-spuser-or-stsadm-o-migrateuser-and-userprofileserviceproxy-value-cannot-be-null/
The solution for this issue in our case was to run the command with the Farm account.