I think this would be a major change, and not a good one.
There are only a few uses for controlled units as it is. First, they
can be used to map out territory. (Only by MOVEs, mind you. They
cannot EXPLORE.) Second, they can sit in places that generate gold --
like inns and castles. Third, they can be used to carry stuff around.
Fourth, they can attack things.
If you limit the movement of men in and out of the controlled units,
you heavily impact uses 3 & 4. Most controlled units are very weak --
on the order of 10 peasants. Without adding additional men to the
unit, a controlled unit isn't really useful for carrying much or
attacking much. And since any men added to a controlled unit still
have to be paid for, it isn't like this represents a huge benefit to
the owner.
I wouldn't argue that controlled units should turn into free nobles,
but I think preventing men from moving in and out of them makes them
too useless. Right now they are nicely positioned between a noble and
an angry mob. I don't think a change would be an improvement, but if
any change is warranted I'd rather see them made less common, rather
than less useful.
-- Scott T.