Originally Posted by Tambreet
I don't know the answer to your first question, but I have been upgraded after boarding before - and let me tell you it is not fun trying to get from the exit rows of an MD80 to F against the flow of traffic.
IME, its best to generally wait until the traffic flow has stopped. Even if they announce for you to come to the front just push the call button, wait, and then go up after the traffic has died down.