White House officials had expected to deliberated for several days over how to respond to North Korea's proposal for direct talks between the countries, which South Koreans officials had first conveyed by telephone this week. But Mr. Kim's offer of a leader-to-leader meeting accelerated, if not upended, the administration's plans.North Korea Asks for Direct Nuclear Talks, and Trump Agrees
Hi, I would like to know the meaning of "if not upended". I looked up the word, upend in a dictionary, and it says:
Set or turn (something) on its end or upside down.Oxford
But still I do not understand it. Please help me.