To Change a mailbox from one type to another in exchange 2007 and 2010 like below :-
User mailbox to shared mailbox
User mailbox to resource mailbox
Shared mailbox to user mailbox
Shared mailbox to resource mailbox
Resource mailbox to user mailbox
Resource mailbox to shared mailbox
Type the following in the exchange management shell.
Set-Mailbox ConfRoom1 -Type value
Where value is Regular, Room, Equipment, Shared.