How did free jazz change the usual rules about harmony and rhythm?

Free jazz fundamentally transformed the conventions of jazz by removing the requirement for adherence to fixed structures.

Regarding harmony, musicians abandoned the traditional reliance on predetermined chord progressions. Instead of following set patterns, improvisers modulate keys at will, often utilizing chromatic intervals, atonality, and techniques such as microtones and tone clusters to prioritize melodic and expressive freedom. This approach, often referred to as "harmolodic theory" by its pioneer Ornette Coleman, allows the tonal center of the music to shift based on the performer's immediate creative choices.12

Regarding rhythm, free jazz performers frequently disregard fixed meters and tempos. Drummers and other rhythm section players often provide accompaniment through pure accent rather than maintaining a steady pulse or direct reference to a specific time signature. In many instances, the traditional roles of soloists and accompanists become fluid, and some ensembles perform without any rhythm section instruments at all, further liberating the music from conventional rhythmic constraints.12