Welcome & Thanks for your interest in Vienna MIR, Till.
Changing the Venue in the XML won't work, as you found out yourself. But even if it would - a myriad of important parameters wouldn't fit, sorry to say so. There's _so_ much going on under the hood, that even in MIR itself the change of an existing setup from one stage to another will need manual adjustments of several parameters like instrument positions and directions, the setup of the main microphone, Room EQs, or dry/wet ratios.
Please remember: Using MIR, we are not just changing some added reverb on an AUX-channel. It's like moving your orchestra to a different hall, with all of its peculiarities. You wouldn't gain much from "just changing the Venue" for an existing and readily mixed arrangement. Room is treated like an instrument within MIR - so this would be like using a solo-trumpet for Mozart's clarinet concerto. ;-)
Kind regards,
/Dietz - Vienna Symphonic Library