I take it your current version is 6.1.2?
Check out the following for supported upgrades:
http://www.cisco.com/en/US/docs/voice_ip_comm/connection/9x/release/notes/912cucrn.html#wp1232121
You can upgrade from the following versions of Unity Connection directly to version 9.1(2):
•9.0
•8.6(x)
•8.5(x)
•8.0(3x), 8.0(2x), and 8.0(1)
•7.1(5x) and 7.1(3x)
So, you will first need to go to one of these versions.
To see if your MCS server can be used as bridge server check out release notes for the particular version you are going to to ensure it is compatible.
HTH,
Chris