I just did this a couple years ago.
The big step is all new task logbook on M2 category aircraft. If you are ACA'd on a M2 category aircraft with your M1 license (thank you transport for creating the huge grey area in license requirements on rotor wing), you are able to sign your own tasks off on your new logbook. I had to argue/discus this with my PMI, but in the end I won.
Quote from Transport task list AWM 566(Skill-4)(e)
"Proof of having completed aircraft maintenance tasks shall take the form of a certification by the AME, or equivalent person who supervised the work. The certification statement shall include ..... Persons who sign for completion of maintenance tasks shall be responsible for the accuracy of statements made."
A letter from your PRM will eliminate all non-applicable tasks.