First and foremost: Do I have a complete backup of all the files, all the attachments, all the databases, and all the custom scripts and other indirect stuff.
Secondly, before any actual migration checklist is considered: Which data am I willing, and not willing, to compromise? Then compare that against what the other software has to offer, and see what is possible or not.
And thirdly, do a test migration in private. And get the team and some select premium members involved to find the unexpected.