Calling ACOM CS is the best method to upgrade your FTM (M). They have the wiggle room to offer you a discount on the upgrade, as well piggy-backing some free subscription time.
Check the current subscription prices and the current price of FTMM before you call, (rates and subscription packages vary depending on what country you're in). If you have a current ACOM subscription; note the time you have left on it, and what level it is; then negotiate away! This can help you get the best deal possible.