The original topic covering the pre-order of Mists of Pandaria as well as Collector's Edition Pet & Mount can be found here.
The new thing is that you will be able to purchase the Collector's Edition as a digital purchase.
The Digital Purchase will only include the previously leaked pet and mount, not the physical box and goods.
For a few minutes, many users got the "Upgrade" screen enabled under their beta accounts.
It's mentioning a Deluxe Digital Edition. This would be the first expansion that's selling the mount/pet along with the game digitally. Prices are yet unknown, but we can assume it will be noticeably cheeper, because you won't get the box, DVD, mouse pad and other stuff which is included in the physical box.
On the left, there's a reminder for people who haven't upgraded to Mists of Pandaria yet.
The digital purchase date is still unkown, though it should be available soon.
Purchasing WoW digitally has its advantages, if you really do not need the physical box.
You are able to download the game beforehand and can play from day one and don't even need to visit a store.