Ideally, such matters should be considered before the publication of the agenda, especially if the change is announced less than the required three clear days beforehand. However, you wouldn't be the first council to amend an agenda after publication and it isn't the greatest sin ever committed. In this example, adequate steps should be taken to ensure that members of the public do not attend the wrong venue or, if they do, there is clear information available to direct them to the new venue. I would be inclined not to hold the public forum at the start of the meeting, in order to ensure that those who wish to participate are given adequate time to arrive.