The change actually makes sense, or we could wind up with more than 12 planets. The discovery of Xena and the others, along with newer telescopes, lead some to predict there could be many, many more Pluto-sized objects out in the Oort Cloud; we could otherwise get to a point where everyone loses track of how many planets there are.
Re: Nine Planets ice cream, that could either be a 'retro' reference back to the era when ice cream first came about, or perhaps there was an artifical planet by that time. Maybe Weber's World originated as a Sol-based world before it was upgraded.