If you sold him the interface and no software, then he just needs to plug it in and go. You will not lose your Protools 9 software and license. They belong to you. Hopefully you let him know that you already used the serial number to upgrade, which means he can't use it for the crossgrade price. However, he can still buy the full version of PT9 and use it. If you sold him the old software with it (PT 7 or 8), and he wants to use it, the transfer of ownership form will help make that smoother. You should still do a transfer of ownership form either way. It will be needed if later on the new buyer wants to take advantage of any hardware trade up programs, like to HD, or if they come out with a 004, etc.