Upgrading to Ultimate Edition
I am afraid it is not. You can only purchase the Ultimate Edition for $100, and..perhaps..apply it to your existing HoT account.
But, if you were going to do that, it would be cheaper to purchase the Deluxe upgrade ($30 in Gem Store, I believe) and $50 worth of Gems.
Good luck.
If you really want the ultimate edition, you can go through the trouble of requesting a refund and buying it instead.