I think what Prime meant in his question is when you're given your packs, before you open them, are you allowed to trade the unopened packs with someone else before they're opened. I would assume that at a Prerelease, since there's no trading until after the event, you should keep the packs and cards you were given and not trade them, even if they've not been opened, until after the event is over.
And as far as the pack swapping for the format goes, it may be acceptable to do it that way at an event, but it just doesn't seem right to me to announce it half way through opening the packs. It seems like that should be something people are notified of ahead of time, so they can decide if that's the event format they want to participate in. It just seems like taking away the fun spirit of a Prerelease atmosphere in my opinion. The opening of the packs for most is the fun part!