You don't have to change management agents but the option should always be there. We were with the same agent for six years. We then sacked them between AGMs because they were no longer working in the development's best interest. (I am a director of our MC with no connections to any management agents)
The power to fire and hire agents is in the directors remit but if you have an issue have it raised as an agenda item at the AGM and vote on it.