Change name to anything, but right before checkout, edit the hidden post fields through inspect element (Specifically new_username), then proceed to checkout. (Be sure to unselect the field so that it is properly applied) Additionally, this may allow invalid characters such as spaces, dashes, etc in the username. Finally, it may be possible to set the last name to anything previously available, such as Linden by setting the new_last_name_id to "1" and new_last_name to "Linden". For me, it takes me directly to checkout, but I did not proceed with a checkout.