Depends on how much discovered it is. Right now, it still possibly does not exist.
If we get more studies with more and different KBOs that confirm the hypothesis, but we have still no chance to discover it by telescopes, we could send a space telescope closer to the region. For example by a highly elliptic (comet-like) orbit, which would allow doing an extended mission in a different region of the solar system with some minor navigation.
If we discovered it, it would just depend on the opportunity - which planets are available for a swing-by, what is the encounter velocity then, could we reduce eccentricity of the orbit by a Neptune flyby to reduce encounter velocity... etc.
As long as we have no good mass estimate of it, only a flyby would work. Even small errors in planet mass estimates would make a useful orbit-insertion hard to impossible. If the errors are small enough, a careful approach would work.
Yeah, Pluto has become the Space Shuttle Enterprise of planets: initially given full status and then having it yanked out from under it, with future schoolkids to be taught it was never a planet to begin with.