Hi,
You should be able to solve this with some editing in the vCenter database. There is one table in the database called VPX_ACCESS. All logins with their corresponding roles are configured there.
Each row has an unique ID, name of the account should be DOMAIN\username, ROLE_ID, ENTITY_ID and FLAG. I am unsure about what FLAG really is. But i will explain the other fields briefly to you.
a) If you want to just switch all the permission from OLDDOMAIN\username to DOMAIN\username you can just edit the rows and change the PRINCIPAL field from OLDDOMAIN\username to DOMAIN\username. This should be enough. Because role_id will tell vCenter which Role is applicable for this user. Entity_ID specifies where in the vCenter structure the user and permission will be effective. Of course try this with a test account first.
b) If you want to run with duplicate permissions for a transition period. I suggest that for each row in the table you create a duplicate row with a new ID and change the value of PRINCIPAL from OLDDOMAIN\username to DOMAIN\username.
If you have many rows in VPX_ACCESS i suggest you read up on SQL or ask some SQL guy to help you with an UPDATE or INSERT query that will perform any of the above. Otherwise it should be quite easy to update / duplicate the rows with SQL Management Studio.
Of course perform database backup before doing any changes and you proceed at your own risk 
cheers