Having completed many migrations and some of them have been flawless and some of them have been complete train wrecks. Here is a list of issues to be focused on.

1. Make a complete backup of all of the data.
2. Communicate with the client about the migration, time, new passwords etc.
3. Understand any special settings for the specific accounts.
4. Make sure you have access to either the accounts DNS and or their domain name to make the DNS changes without any delays.
5. Do not switch DNS on more accounts than you can handle.

I hope this helps.