G Suite to O365 migration

Occasional Visitor

I would appreciate some advice or insight on how to best handle a migration from G Suite to O365.  Here's my situation:


Company A has about 350 G Suite accounts with email users set as username@companyA.com.  Company A was acquired by Company B, which is using O365 mail, accounts are username@companyB.com.  All of the Company A users ALSO have mailboxes in the CompanyB O365 domain and both mailboxes for those people are actively in use.


I've been tasked with migrating mail, calendar, contacts, etc. from Company A to Company B.


I've reviewed the Microsoft docs on both G Suite migration and IMAP migration.  The sticking point is the target users in the O365 domain.  The docs recommend using subdomains for each side of the equation, and setting up mail aliases in G Suite and provisioning MailUsers in O365 with an external mail address pointing to the G Suite subdomain and a mail alias for the O365 subdomain.  The migration process (performed in EAC) then converts MailUsers to mailboxes.  Once those O365 mailboxes are created, the user is supposed to stop working in G Suite and use O365.


My uncertainty lies in the fact that all the users to be migrated already have a mailbox in the O365 domain.  I've considered doing just an IMAP migration because that requires O365 mailboxes be provisioned before the mail items are transferred.  But the IMAP method moves ONLY mail items, no calendar or contacts.  I REALLY don't want to have to do PST imports for contacts and calendars, since almost every staff member is working remotely due to the pandemic.  


To throw another variable into play, Company A also has an O365 domain, with no mailboxes.


So here's my query:


DO I do the G Suite migation method so I get all the items in one fell swoop, or will it fail due to the prior existence of mailboxes for Company A users in the Company B O365 domain?


DO I do the IMAP migration, and then come up with some way to manage migration of calendar and contact items for those G Suite users?


Do I migrate Company A G Suite to Company A O365, and then Company A O365 to COmpany B O365?


I'd appreciate feedback for anyone that's had a similar set of circumstances.  THANKS!

1 Reply

@MPerryDS I had previously used Quest Zero touch migration manager and looks like they have a specific too for G suite migration https://www.quest.com/solutions/google/ where you could transfer from a source gsuite to a destination O365 account with contacts, mails etc. Hope this helps!