Hello HT686
This could be done in two simple steps with Powershell / Exchange management Shell
First we disable/Disconnect the Invoice mailbox
Disable-Mailbox -Identity "Invoice@info.net"
Then we delete Johns mailbox
Disable-Mailbox -Identity "John.smith@invoice.net"
After this, we will now reconnect the Invoice mailbox, but to Johns AD user
Note that "MXDB01" Will need to be replaced with ypur databse name.
Connect-Mailbox -Identity "Invoice" -Database "MXDB01" -User "John Smith"
Let me know if you need further assistance
Kind Regards
Oliwer Sjöberg