Also, it makes a difference if you ordered from Apple. If you ordered a wifi + 4G from Verizon or AT&T, they'll want to charge you a re-stocking fee. Apple doesn't. That was a big reason why I pre-ordered from Apple (I ordered two 64 GB wifi + 4G one has AT&T the other VZ; one will be returned in-store after my side-by-side hands on during the 14 day return period).
And the last sentence is the key--available stock (other than a like kind exchange due to defect) could be sparse in the 14 days after March 16, depending on what you want. Worse case, you end up with a return, a credit to your credit card, and needing to wait for a new one with larger capacity.