Yes you can upgrade it to whatever image you need (H.323, SIP, SKINNY or MGCP). You need to download the appropriate image off CCO. They are supplied as .ZIP files and inside are instructions on how to upgrade. Basically you extract the files to a PC and execute the server upgrade application. This acts a bit like a tftp server (but not quite...). THe ATA must have an IP address and be able to reach the PC. With a handset plugged into port 1 on the ATA you lift the receiver and press the button on the ATA, you then enter the following digits on the phone:
100#<ip_address_of_PC>*8000#
The IP address octets are separated by '*', i.e. if the PC's IP address was 10.1.1.1 you would enter:
100#10*1*1*1*8000#
The instructions are included in the .ZIP file.
Good luck
Andy