You want 'vsx_util change_interfaces' on the management server. You will need to take an outage, but it's a pretty mechanized process. Basically, you define the new interface in the VSX object's list of physical interfaces, publish, then run 'vsx_util change_interfaces on the management server. It will prompt you to log in, then ask which cluster you want to change interfaces on. It will ask you which interface you want to replace, then which interface you want to replace it.
I highly recommend building a bond which contains the new 10g interface and changing VSX to reference the bond. Then changes like this in the future are a lot easier (you just change which interfaces are members of the bond).
Run it in a lab a few times before doing it for real. I haven't had to do this in a while, so I may have forgotten a step.