The first step is there was no notice, which may invalidate the change, but that depends on the circumstances. You are required to put any proposed resolution on a notice of meeting.
Ask for the minutes of the AGM and check the resolution for the by-law, if it wasn’t on the agenda, who moved the motion? What was the vote on it? (this should be recorded)
A change to a by-law or a new by-law requires a special resolution, i.e. 75% of those voting at the meeting either in person or by proxy. The thing is, if someone gave a proxy and there was no notice of the proposed by-law change, how could a person giving the proxy give instructions?
Finally – has the by-law been registered. If not it has no effect.