I am also looking for an answer to this as I purchased the regular edition when first announced but have reconsidered and would like to upgrade to the Collectors edition... Please tell me its possible!
There is no official answer that I know of, however ZOS did sell the original games collectors edition as an upgrade later.
Unless you hear from ZOS directly anywhere your safest bet is to buy the collectors, remember that you get the preorder bonus as long as you order before June 6th so if you need some more money you can still save up and buy it in may to get the preorder bonus.
@Turelus - EU PC Megaserver "Don't count on others for help. In the end each of us is in this alone. The survivors are those who know how to look out for themselves."