it depends on the promotion. the ac21 stipulates "New employment must be the 'same or similar' occupational classification". According to my coworkers, that means they can't get promoted to management from dev/qa.

http://immigrationroad.com/green-card/ac21-portability-chang...

no it won't. According to my understanding this is what happens

a) if you have applied for 485 and it has been 6 months since you applied you invoke AC21 and you are done.

b) if you have not applied for 485, your new company will have to reapply for 1-140 and labor for the new position but you do not lose your place in the queue as your PD is ported from your previous application.