You can once the game is released on 4/4/14. Otherwise, I don't believe you can without canceling your previous order and re-ordering the imperial edition.
Keep in mind that apparently you cannot add an Imperial Edition early access code on top of a Standard Edition early access code. You will get the error message "you already own this product" and will need to contact support to have this fixed.
I do not know if you can add an Imperial Edition retail code on top of a Standard Edition preorder code.
Once the game is officially released they will have an ability in game to upgrade your version to Imperial Edition, or if you simply want a cheap horse they are selling a different color horse for an undisclosed amount. This will not however go live till 4/4/14,