Really? You'd think the seemingly plug and play nature of the way characters behave within missions it'd be a matter of a simple patch, not a total recompile.
For a homegrown engine, perhaps. But apparently the modern Unreal Engine (2.5/3) does a lot of precompiling/optimization of assets, and doesn't allow for that kind of runtime swap. It's the same reason why the Bioshock 2
devs couldn't fix the silent vending machines via patch - they'd need to include recompiled versions of all of the levels in the patch.