Once the agenda has been published then the meeting must go ahead even if it is unlikely the meeting will be quorate. Someone has to go to the meeting venue at the due date/time and if necessary announce the meeting can't start.
If the agenda hasn't been published, then yes the meeting date can be changed but the requirement for three clear days notice still exists so if the date is changed to the 14th, then the agenda still has to be published no later than 8th or 9th (depending on whether you count Saturdays).
Sometimes it is necessary to change a date of a scheduled meeting, particularly if it appears the original date is unsuitable as suddenly people can't attend or there's a problem with the venue. The only requirements with regard to changing a meeting is that the three clear days notice must be given for the revised date.