Would it be practical to institute a rule that once a pilot has flown an Airbus they can't commercially fly a Boeing?..
Do pilots transitioning from the Airbus to the Boeing get put through an intensive simulator training to '(re)acclimatise' them to the Boeing.
By intensive I mean going through a whole host of failure modes.
For instance, I picked a random link to a flight school that offers a B777 type rating course[2] - which takes 14 days of ground-work and 9 days of flight training (36 hours in the simulator in this case; often for big airliners it's going to be too expensive or impractical to perform a type-rating in a real aircraft).
Additionally, having attained the type rating, you're required to maintain "currency" in that type by flying it frequently; if you let type currency lapse by going too long without flying it, you need to undergo a kind of mini "refresher" training/assessment to regain currency again. This is the case for Private/GA Pilots in NZ at least, I'm not so familiar with other jurisdictions or license categories.
[1] http://en.wikipedia.org/wiki/Type_rating
[2] http://www.balticaa.com/en/aviation-training-courses-/type-r...