There was a published article on the steps at support.avaya.com. With the new servers the Maintaining guide for S8730, for example, on page 14-15 shows the procedure. It basically says to busy-out the ports for each MAS before shutting down. A good rule of thumb is also to Suppress Alarms on the MAS and if it is a MSS backend to Activate Alarm Suppression so these alarms don't go out to a NOC center. After the MAS (starting with MAS1, then MAS2, etc) comes up, open the ports and uncheck Alarm Suppression.
Hope this helps.