Reading this question about multiple gravity assists (Why is Voyager 1 faster than all other space probes?), it caused me to wonder if any probes have been redirected to additional (or alternate) destinations other than what was planned when they were launched? I'm not wondering about rovers on a moon/planet, but rather spacecraft themselves.
Back in 1983 the International Sun-Earth Explorer 3 spacecraft (ISEE-3) was diverted, via a very small ∆V and Earth-moon gravity well manipulations, from its Sun-Earth L1 halo orbit to a rendezvous with comet Giacobini-Zinner in 1985, summarized in this popular article.
Yes. The primary mission of the New Horizons probe was to do a flyby of Pluto in 2015, after which funding was secured to look for an appropriate Kuiper Belt object to target for another flyby. The probe's maneuvering capability was quite small, so it was challenging to find something near enough to its trajectory that it could divert to, but it performed a flyby of 486958 Arrokoth (aka Ultima Thule) in 2019.
Yes. The THEMIS mission, launched in 2007, was designed as a constellation of five Earth-orbiting satellites in coordinated high earth orbits (with orbital periods of 1 day, 2 days, and 4 days). After the two-year prime mission, the orbits of the two outer probes would have been perturbed away from the regions of scientific interest in the Earth's magnetotail. It was realized that the outer probes had enough propellant remaining to perform a complex series of maneuvers, that would eventually put both of them in lunar orbit, where they continue to operate to this day. Every month, the Moon pulls them through the Earth's far magnetotail, where they can make coordinated observations with the remaining three Earth-orbiting probes, while monitoring the solar wind or the near-lunar environment the remainder of the time.