When do I need to set up an 1031 exchange

[Qi Ming Chen]

I am under the impression that I only need to use a 1031 exchange before the closing of sale. At the moment I am already in contract to sell a property and I heard from someone today that I should have sold the property under the 1031 exchange, is that true? If so does that mean I now have to modify the sales contract and get the buyer to sign it again?