Depends on if people have paid money to attend, and if you have sponsors on board already for it. I have had to cancel (actually postpone as it turns out) a free to attend event, paid for by an event partner. The conversation was difficult but if people realise it's in their best interests they should agree to it. Where the partner/sponsor has committed to their organisation to deliver activity in a particular Q, then things become more challenging though. Attendees will come and go - many will have forgotten that the first event was cancelled in the first place!