A similar thing happened with my wife and I. We arranged wedding in the US and filed the K1. Unfortunately the wedding came just before receiving the letter to arrange medical and interview. We already spent a fortune in a US wedding, had family coming from all over the world and we simply couldn't cancel, postpone or turn it in to a party ceremony with the intention of getting married in a court room at a later date as it wasn't fair on anybody (especially those who booked flights, hotels etc from afar) to go all the way across the world to a wedding which, well might not even be an official wedding!! I mean if I travelled across the world for somebody's wedding... Spending a fortune I don't think I'd be too happy if I found out it was just basically a party and not even an official ceremony.
We got married, sent a cancellation letter to uscis and filed for the I-130 and are currently waiting all over again!
Very painful after being so close to getting the k1 but that's the way it is unfortunately. At least my conscience is clear