I cannot ever foresee a situation where we'd want you to follow the old missed approach, hence we would always give you appropriate instructions.
I think mandating that you have to follow the new runway's missed approach procedure is unworkable, as some switches happen very late, within four miles. Are flight crew really going to brief the new missed approach procedure in that time?
What we expect, and what we want (and then what happens) are two very different things.
We expect you to follow the old runway's procedure, because that is the 'worst case scenario', so that we (ATC) do not get complacent and believe you will follow the new runway's procedure.
@londonmet, the point is that ATC would act in your case. As it stands, a normal missed approach from 27L does not separate you from 27R departures. ATC still has to act to separate you. So a similarly serious situation could happen in normal ops, if ATC don't intervene...as we are all taught to do.