Special resolution
Not all decisions are equal. Ordinary business passes on a simple majority; changing the constitution usually does not.
A special resolution requires at least 75 per cent of votes cast, and the notice convening the meeting must set out the resolution and state that it is proposed as a special resolution. Getting the notice wrong invalidates the resolution even if the vote was overwhelming.
Check what your governing document requires and what notice period applies — see members and general meetings and written resolution.