You can convert mailbox from one type to other type in two ways

1. Throught Powershell command

     Set-Mailbox <mailboxname> – Type <type>

     Type can be Regular, Room, Equipment, Shared

2.  Second option is throught Editing Value of msExchRecipientTypeDetails  from ADSIEDIT.

      Access user properties throught ADSIEDIT and find the attribute msExchRecipientTypeDetails change values. Values for Different mailboxes is given below

     User Mailbox : 1
     Linked Mailbox : 2
     Shared Mailbox :4
     Legacy Mailbox :8
     Room Mailbox   : 16
     Equipment Mailbox :13