change gear begin to move or act differently, usually more rapidly.

☞ This expression derives from literally engaging a different gear of a motor vehicle in order to alter its speed. Compare with in gear (with a gear engaged, and so ready for action) and its opposite out of gear. To move up a gear means literally 'change to a higher gear'; the phrase is often used figuratively to mean 'put more effort into an activity'. To go (or move) through the gears is metaphorically to 'operate with increasing force or intensity through successive stages of an activity'.