The upgrade is normally possible, but you, of course have to pay the difference in setting up price and tier, I was just wondering if the promotion could be applied in this case also, since, as you noticed, there is no mention of this particular case... but they also don't state that it is not possible, so you'll never know =)
I was hoping somebody asked this already and knew the answer, since I can't seem to get a reply from LL today.