The change is unnecessary. While a specfied number of dimensions and worlds was never revealed, it does not automatically make it below the 1000 mark unless something stated otherwise. How many worlds Super Dimentio is going to destroy is irrelevant because he's going to destroy all of them and all existence. Basically, that's everything that exists, including dimensions that exists between dimensions such as
Flipside and the afterlife such as
Underwhere and
Overthere (Basically, Hell and Heaven). Even worlds that aren't explored in the game and yet to be discovered in the series.
The verse of Mario is constantly expanding. In Luigi's Mansion

ark Moon, there turns out that a world of ghost exists called the
Paranormal dimension and Mario and Luigi: Dream Team, the gimmick is travelling btw the Real World and Dream World, and mutiple of them exists even outside the game such as
SMB2's Subco and
Mario Party 5's Dream Depot. This goes even further when you take in the population of the Mushroom Kingdom since the Dream Depot houses dreams of the MK inhabitants, which is clearly over 1000 populants. And then, you have worlds from sub-series such as the
Shake dimension from Wario Land: Shake it!, the dimension where the player fought Behemoth and Behemoth King in
Mario Sport Mix and
Vanda(Unnamed in Japan), a universe where Culex originated from in SMRPG. That being said, the rating stays. Plus, I'm certian their are plnety of characters rated 2-B for similar situation, making it a double standard for Super Dimentio downgraded. It going to take more than the lack of total number of universes to downgrade a constantly expanding verse.
@ Kevyn Souzan
Cameos and guest appearances shouldn't be used to connect seperate franchises as a single verse. After all, Mario starred with Sonic several times in the Olympic games. That doesn't mean, they are from the same verse. With that being said, all Nintendo characters are within the verse they originated from and not from the camoes or easter eggs they make unless the settings of two franchises/characters/etc is clearly within the same verse eg. The Donkey Kong and SMB franchise.