Hi Darrell,
This is Neo Zhu from Office365 support.
Based on your description, your customer has the local Exchange environment system. However, they don’t want to migrate the existing mailbox to Office365. The problem your customer have is that after the dirsync re-enable , you worry about the customer can’t create the mailbox again.
I’d like to answer your questions first.
Q1. At this point, will DirSync soft match the on-prem account with the O365 account? The primary SMTP address is the same between on-prem and O365.
A1. Yes, soft match still works when the SMTP address match. support.microsoft.com/.../en-us
Q2. Is there a way to get DirSync syncing now, while the on-prem Exchange server is running, to control the O365 accounts from on-prem AD? Would that involve deselecting a list of Exchange attributes in the Active Directory Connector Management Agent, in the Synchronization Service Manager? If so, what attributes should I deselect?
A2. I think a workaround solution. However, I doesn’t test yet. You can see the detailed information below:
1. Disable local mailbox user (disable-mailbox)
2. Re-sync
3. Enable Office365 licenses and make sure the mailboxes are created on cloud side.
4. Enable local mailbox (enable-mailbox)
If you have any questions, please feel free to post.
Best Regards,
Neo Zhu