Pluto has been reclassified as a Dwarf Planet, which means that it is a celestial body large enough to be controlled by gravitation, but not large enough that it has cleared its orbital region from other objects in the solar system. Pluto’s orbit is quite erratic – wide and oval rather than circular, and it crosses into Neptune’s orbital path. One surprising fact about Pluto is that although it is large enough to have multiple moons, its surface area is actually smaller than Russia (Which is absolutely minuscule for a celestial body, comparatively speaking). There are 5 Dwarf Planets in all: Ceres, Pluto, Haumea, Makemake, and Eris.
Ceres was previously identified as an asteroid as part of the Asteroid Belt between Mars and Jupiter. Pallas, Vesta and Juno are still identified as asteroids. Those who have not read the manga may not know this, but the Amazon Quartet (CereCere, PallaPalla, JunJun and VesVes) become the Sailor Quartet, and serve as Sailor Chibi-Moon’s guardians in the future. Sailor Ceres exists, therefore Sailor Pluto can also exist.
Being classified as a planet has never been a prerequisite for becoming a Senshi. You know that character, Sailor MOON? The Moon is not a planet! It’s a satellite. A satellite is a celestial object that orbits another larger celestial object. Theoretically, any celestial body, whether it is a Planet, Dwarf Planet, Moon, Star, Asteroid, Comet, etc, is capable of having its own Sailor Senshi.
Taking all of this into account, plus the fact that Sailor Pluto is significant to most of the Sailor Moon story, absolutely nothing will change regarding her character, dwarf planet or no dwarf planet.
(And yes, there are still raging debates as to how to classify Pluto. Science is, by nature, eternally changing and questioning. A person who lives outside Time has better things to do than worry about scientific squabbles anyway!)
Artwork from Artbook III, scanned by SailorMusic.Net