Before you start
Sometimes the answer is a conversation, not a change
Changing agent takes effort, and it is worth being honest about whether it will fix the problem. If the issue is one manager rather than the firm, asking for a different point of contact sometimes solves it. If the issue is that the service charge is not covering what the building needs, a new agent will arrive at the same conclusion the old one did.
Where it usually is worth changing is when the pattern is consistent: repairs that never get closed out, accounts that arrive late or do not reconcile, compliance that has quietly lapsed, contractors who seem to be chosen for reasons nobody explains, or a firm that simply does not respond. If you have raised the same thing three times and nothing has changed, it is not going to change.
We would rather tell you the problem is fixable where it is than take on a block under a false expectation.