A majority of the Parent Forum can change the constitution of the Parent Council at any time. The constitution should be reviewed from time to time and the Parent Council can ask the Parent Forum to make changes. Changes can only be made by a majority of the Parent Forum - voting would be a good method of idetifying the majority view. In practice, it is a good idea for such changes to take place at the Annual Meeting.
What your constitution might say
The Parent Council may change its constitution after obtaining consent from members of the Parent Forum. Members of the Parent Forum will be sent a copy of any proposed amendment and given reasonable time to respond to the proposal.