Strictly speaking, a sectional change is not a gear change but a step in the structure of the song, such as from the cuerpo to the montuno. Such a change often coincides with a new gear, though.
A timba song starts with the cuerpo: the actual song, with verse(s) and often a bridge. On a studio recording that is about 20–30% of the track; live it can be much shorter. Then comes the montuno, the open part with coros, mambos, breakdowns and solos, and finally the coda.
Most gear changes happen in the montuno, sometimes as often as every four claves. Each change can usher in a new section:
Some section changes are fixed in the arrangement. Many bands keep the montuno open and flexible: the singer decides live, with hand signals, which section or gear comes next.
A new section is a natural moment to start something new: a new figure, a different style, or dancing apart from your partner for a while.