This is at the 365 level, not the Exchange level. To connect:
Connect-MgGraph -Scope User.Read.All
(or User.ReadWrite.All)
To get user info:
Get-MgUser -UserID username@domain.com
To change immutable ID to “a”:
Update-MgUser -UserID username@domain.com -OnPremisesImmutableId "a"
One cannot change ImmutableID to $null, but it can be changed, so an AD/AZ ‘hard match’ is probably still possible. Have not verified yet.
The Exchange Online direct connection from Powershell, does remain, and is freshly updated.