If you want to move a business from Microsoft Office365 to Google Suite, there is a nice migration utility in Google Suite Admin Console that helps you transfer all accounts data from Exchange Web Server or IMAP.

In case you encounter an error here goes a list of error codes and troubleshooting guide [1].

I did a full migration by granting my temporary user Global Administrator right by current Admin user, then in Exchange Admin Console I had to grant myself an ApplicationImpersonation Admin Role (to access other accounts for migration). After the migration was complete I have rolled back my permissions to a minimum level.

[1] https://support.google.com/a/answer/6254288