So here is how I do it usually:

1. Call M&M Cust Service and ask for upgradeable seats for the scheduled dates (they will tell you how many upgrades are possible on which flight)
2. Call my TA and buy the ticket, telling him/her to issue the ticket immediately (sometimes TAs only book your flight, but do not issue the ticket immediately. Without an issued ticket, you will not be able to upgrade.)
3. Call M&M again and book the upgrade.

Always worked for me.