*Officially* upgrades should, I believe, only be one cabin. This means, in your case, World Traveller to World Traveller Plus. I wouldn't sniff at that.
"Upstairs" is Club World, and very nice it is too. The word "Club" does actually have some meaning if you can get seated here. There are Club World seats on the main deck and they are equally comfortable but somehow, upstairs is just so much more intimate. That makes it all sound like something out of this world. Remember, you are on an aircraft!
If you do get the upgrade enjoy it.