I don't know about how it works on PC or steam but console if you only buy the standard editions but wish to upgrade to the CE ones at a later date, you can purchase them from the mog station without any problem. Now it would be silly if it wasn't the same for non console users, but I did hear that there are registration issues if you don't buy all the stuff through one thing, i.e. the expacs all through steam if ARR was from steam to begin with etc.

Hopefully someone can confirm for you soon.