You never bothered to update us on the outcome of this by the way. We can all help each other and make this forum much more useful by not only asking questions about issues we’re dealing with, but by providing updates also.
Yes, 1.5 months after I sent my withdrawal letter for early filing to local USCIS office, I got a letter stating to come to USCIS and it didn't say that its for Interview. It stated that its a matter concerning my AOS application. The date was february 26 th. Both of us went to USCIS. Officer came and took us inside and said that its the interview and then only we came to know. Although we brought all necessary documents, just in case,except medicals.