So does that apply if you change an existing pre-order? I've been pre-ordered since they released the order codes to the game stores
It only applies if you fully pay for the game, so it's considered a pre-purchase, instead of a partial payment, which is considered a pre-order.
Pre-orders only get 1 day headstart, and no beta access. As mentioned in my earlier post, Prepurchasing from today on, for any version, gets you full beta access on the 'open beta' weekends, and 3 days headstart, plus a bonus accessory that boosts your character's stats a bit.
If you preordered from a company such as gamestop or best buy, you can fully pay off the balance of the game's cost to 'upgrade' from preorder to pre-purchase, which comes with a snazzy box for the beta access code.
(And yes, some Gamestops or EBgames have not yet recieved the SKU and beta boxes yet, much to the clerks' chagrin)
If you're aiming for the collector's edition, remember that it's a ~very~ limited run; A-net is actually losing money on each CE sold because the 10" Statue of Rytlock is worth 120$ alone.