I have just done it in my lab, running 10.5.2.10000-5. You can simply change it by issuing the command:
utils cli set-cluster mixed mode
Restart the TFTP and CallManager services, you are good to go!
Detail post here:
http://www.cisco.com/c/en/us/support/docs/unified-communications/unified-communications-manager-callmanager/118893-technote-cucm-00.html
