Dissociating an Exchange mailbox with it's AD user / changing the AD user

We should firstly disconnect the mailbox (this will not delete the mailbox immediately - but it will be marked for deletion - 30 days by default.)

Disable-Mailbox "user@example.com"

This will remove all of the ad-related attributes with the mailbox.

We should then connect the mailbox again - ensuring we use the '-User' switch to direct it to our new user (else it will automatically attempt to pickup a user from AD)

Connect-Mailbox -Database "Mailbox Database Name" -Identity "Old User" -User "newuser"

*** Where "Old User" == the display name of the mailbox.