After coming across this thread thread955-785016 which sort of relates to this issue, I thought I'd post some additional blurb I found and used successfully in case others are left scratching their heads on what used to be such a simple issue to resolve with 5.5 
Instead of selecting a new Primary Account user and hitting a button a la 5.5, with Exchange 2003 you effectively have to detach the mailbox from the current user account then re-attach to the required one.
First thing to do is check you mailbox retention settings. Locate your relevant Mailbox Store in Exchange System Manager, right click and go to Properties -> Limits tab. In the Deletion Settings area make sure there is a value greater than 0 in the 'keep deleted mailboxes for (days)' field.
To detach the mailbox, open the Active Directory Users & Computers console and right click -> Properties. Select Exchange Tasks then either Delete Mailbox or Remove Exchange Attributes. Confirm as required and wait for the Recipient Update Service to run. Also, in Exchange System Manager navigate to your Mailbox Store, select the Mailbox folder, right click and select Run Cleanup Agent.
You will see when the mailbox has been detached by the red circle and cross icon next to its name. It's now time to re-attach the mailbox to a different user.
In Exchange System Manager, select the appropriate mailbox (with the icon) and right click -> Reconnect. Specify the new account you wish to link the mailbox to then wait until the RUS runs again.
And that should be it! The mailbox has been transferred from one user to another.
Hope this proves to be useful to someone in need![[thumbsup2]](/data/assets/smilies/thumbsup2.gif "[thumbsup2] [thumbsup2]")
Instead of selecting a new Primary Account user and hitting a button a la 5.5, with Exchange 2003 you effectively have to detach the mailbox from the current user account then re-attach to the required one.
First thing to do is check you mailbox retention settings. Locate your relevant Mailbox Store in Exchange System Manager, right click and go to Properties -> Limits tab. In the Deletion Settings area make sure there is a value greater than 0 in the 'keep deleted mailboxes for (days)' field.
To detach the mailbox, open the Active Directory Users & Computers console and right click -> Properties. Select Exchange Tasks then either Delete Mailbox or Remove Exchange Attributes. Confirm as required and wait for the Recipient Update Service to run. Also, in Exchange System Manager navigate to your Mailbox Store, select the Mailbox folder, right click and select Run Cleanup Agent.
You will see when the mailbox has been detached by the red circle and cross icon next to its name. It's now time to re-attach the mailbox to a different user.
In Exchange System Manager, select the appropriate mailbox (with the icon) and right click -> Reconnect. Specify the new account you wish to link the mailbox to then wait until the RUS runs again.
And that should be it! The mailbox has been transferred from one user to another.
Hope this proves to be useful to someone in need
