Either of you can change any decision. The moment you do, the agreement is no longer accepted, and it needs accepting again by both of you before your plan follows the new rule.
This is deliberate. Somebody changing their mind is the system working, not an alarm. But a rule that one person changed alone is not an agreement, so your plan will not act on it until you have both said yes.
Facts are different
Updating a salary, adding a bill or recording a new commitment takes effect straight away. Those are facts about your situation rather than rules about how to treat them, and they do not need renegotiating.