A gear is about the rhythm section, not the harmony. Still, the harmony sometimes changes along with a gear change, for example when a new coro starts or when the piano almost drops out in a sparse gear.
The montuno usually runs on a short, repeated chord progression, carried by the piano tumbao and the bass. Precisely because that harmony is so stable, a change stands out:
You hear a harmonic shift more as a different "colour" than as a different rhythm. It is a good moment to adapt your styling to the new mood.