In principle, the same formality is required to change an agreement as to make an agreement. For instance, if an agreement is made in a group meeting and properly documented, no member of the group should assume that the agreement has changed or act in ways contrary to the agreement until the agreement is changed in a group meeting and properly documented.
Group agreements often get ignored over time and people come to think it is okay to behave contrary to what was agreed to. The problem with this is that expectations become unclear, especially for newcomers. People are navigating according to different maps — some written, some imagined — and many people are lost. There is inefficiency, frustration, and resentment.
Practical Tip: Make group agreements with a degree of formality and write them down (in meeting notes or otherwise). When it becomes apparent that people are behaving contrary to an agreement there are two choices: Point out how behavior is contrary to an agreement (people may simply not know) and, if contrary behavior continues, enforce the agreement by imposing consequences on violators. Or, formally change the agreement to be in line with behavior. Things change and sometimes agreements need to change, fine.
Ignoring, condoning, or practicing disagreeable behavior is not a choice; not if you are striving for harmony, productivity, and efficiency in your group.