Guys, please don't take ANYTHING the men and women in the store tell you for granted!
In 99% they are wrong about hardware facts, simply because they are not trained the way they should be. It makes total sense that they don't know anything about the RAM of the new MBA, yet (or ever).
Fact is, the RAM can't be upgraded in the complete MBA line. As said a hundred times in this forum, it is soldered to the logic board! There is absolutely no way that an Apple Store (even if they are also an AASP) can replace the RAM chips. They'll swap the whole logic board in case of a failure of any system part.
In case that a user that already has a 2GB model wants to upgrade his RAM to 4GB, they will say that it is not possible (maybe not as of now, but they will once they spoke to their supervisor who may have better understandings of the current hardware).
And since Apple will certainly not swap the logic board for a RAM upgrade which, by the users' idea, should be done for $100, people that want to go up from 2 to 4GB have only one single option, buying a new MBA or pay the price for a new logic board, which btw. will be very close to the retail price of the whole MBA.