If you need to revert to CUCM, the process is identical. Simply download the SCCP firmware from Cisco and place it on the TFTP server. The phone will downgrade/upgrade as needed. Keep in mind that once converted to SIP, the phone loses "Enterprise" features like Cisco Extension Mobility.
Since you cannot download legally without a contract, here is the : cisco 6921 sip firmware download
To download the SIP firmware for a Cisco IP Phone 6921 , you must access the official Cisco Software Download If you need to revert to CUCM, the process is identical
If you have a valid contract, go to: