Firstly, apologies if this is addressed elsewhere but couldn't find the info! I pre ordered the standard edition of the expansion as soon as it was available, but after extras were added I understood we would be given the opportunity to upgrade to the collectors edition. I haven't seen any reference to how to do this and preorder is about finished. I'm hoping the 'top up' will be the difference between the standard edition and the discounted pre order price for the collectors edition, not the full price. It wouldn't be fair to have to pay more just because SOE added items partway through the pre ordering period! Perhaps I missed something?
YOu can simply cancel your normal preorder addition and put in a new order or CE, before time runs out.
I've seen another suggestion on the boards to cancel your current order and setup a new order for the CE. I haven't seen any postings about actually upgrading it.