This is totally different. What I am describing is more akin to buying a 16 GB iPod touch and then exchanging it for store credit which you use to buy the currently out-of-stock 32 GB model a couple of weeks later.

I don't have the specific documentation that says this is the case, but I personally canceled my 3G iPad preorder and bought a WiFi model yesterday at the advice of an Apple Store employee.

Just call your local Apple Store and they will verify it for you.