My fiancee is scheduled for K1 visa interview at the U.S. embassy, can she walk in and inform them that we got married or shld I start a k3 process?

My fiancee has an approval for k-1 VISA and she is scheduled to to have the interview in August,2020. Due closure of the U.S. embassy due to Covid-19., her interview was delayed from April to August. We were suppose to get married here in the U.S. and I have prepared everything for the wedding in July. Now due to the delay and certain circumstances, we must get married overseas. can I convert a K-1 visa to K-3 or should she walk to the interview and inform the officer about she has got married already? Shall I start the process from the beginning to obtain a k-3 visa after the wedding over seas? Can I contact the USCIS and they can convert it for me since they already have all information about me and my fiancee?