Exchange AutoDiscoverInternalURI after 1 of 2 domains migrates to MS365

Question

On-prem Exchange is authoritative for 2 domains. One domain is migrating to MS365 via Cutover Migration as documented here--

https://learn.microsoft.com/en-us/exchange/mailbox-migration/cutover-migration-to-office-365

The other will remain on-prem for another few months, then also move to MS365 the same way.

The current AutoDiscoverInternalURI is--

https://mail.<DomainThatWillRemainOnPrem>.com/autodiscover/autodiscover.xml  
  

Outlook will continue to be used as the email client for both domains. Some users will use Outlook against the 365 domain, some against the on-prem domain, and all users will be on the LAN.

The Cutover Migration article says to issue this command as a "Post-Migration" task--

Set-ClientAccessServer -Identity <ServerName> -AutoDiscoverServiceInternalUri $null  

Trying to figure out if I really want to do this while we're half on-prem & half 365.

What are the ramifications for the on-prem domain if I do this?

What are the ramifications for the 365 domain if I don't?

TIA

Answer 1

I would not change the autodiscover entry until ALL the on-prem mailboxes ( and any public folders) are migrated to 365.

If you do, you risk breaking the existing on-prem mailboxes accessing Exchange remotely.

There is no risk in not changing the record for 365 mailboxes. Modern Outook clients use direct connect to Exchange Online anyway:
https://learn.microsoft.com/en-us/outlook/troubleshoot/profiles-and-accounts/unexpected-autodiscover-behavior

Older clients will still use the on-prem SCP if necessary and then get redirected to 365

Answer 2

Thanks @Andy David - MVP for the quick answer!