This depends on whether the message is a scheduled message (based on check-in / check-out) or is a reply to an event (such as a new reservation or a new inquiry messaging rule).
Scheduled messages
Automatic messages that are scheduled to be sent relative to check-in/check-out are generated for guests at the moment that a booking is confirmed.
If you make changes to a messaging rule, and a message is already generated for a guest, our system will update all pending messages for every future and ongoing reservation.
The following elements are updated in the manner described below:
Changes to the message template will update pending messages.
Changes to the scope (host/platform/property) of the messaging rule will update pending messages.
Changes to the timing will update pending messages.
Changes to the specific rule settings (depending on the rule) will update pending messages.
If you have manually edited (or simply clicked on 'Save' without making changes on) a pending message to a guest, then it will not be updated by any subsequent changes made to the messaging rule or reservation's dates/time of check-in/check-out. Since the message has diverged from the original template, this message will not be updated.
The same apply to messages that are cancelled; they will not be regenerated and will remain canceled.
Replies to events
Automatic messages that are generated by events such as new inquiry, new reservation and new cancellation will not be updated when you make any changes to the messaging rule, even if the message has not been sent yet (this means its Timing was set to either prepare for sending manually, or delay by an amount of time).
This also means that if an event messaging rule Timing was originally set to prepare/delay and you change it to send immediately, previously generated messages will not be sent immediately.