If nothing else, you can point out at the meeting that it is normal to have the agency agreement included in the meeting notice so that people have a reasonable change to know what they are agreeing to. We include the agreement even when renewing every few years with a manager we are generally happy with. A few years ago an owner noticed a slightly odd clause and with the manager present at the meeting the general meeting was able to resolve to accept the agreement subject to an amendment to that clause. It was not a big deal but only possible because we distributed the agreement with plenty of time for people to read it.
It is usually easier to scare people with the unknown than to convince them of a known change so you don’t have to prove anything bad about the new manager or suggest that the old manager did not deserve to go. You only need to argue that the meeting does not have all the information it needs and a reasonable time to consider the information in order to make an informed decision.