You order game, before release you get an EA code that allows you to download game and to play during a set amount of time.

When that amount of time runs out if you don't input the CD key that came with the physical copy you can't play the expansion.

That's one way it's been handled when it's come to EA and I think they even did that with the 1.0 release. If I recall correctly for EA you got a code on your store receipt and with that code you could gain entry into Early Access.