变卦 is a separable verb (离合词) meaning 'to go back on one's word' or 'to change one's plan unexpectedly'. The first part 变 ('change') and the second part 卦 ('divination trigram') can split when you insert aspect markers, measure words, or time words: e.g., 变了卦 ('changed plans'), 变过卦 ('has changed plans before'). The literal origin involves changing a divination result, so the word carries a sense of a sudden or drastic shift — it can also describe a situation that 'takes a turn.' Use it when someone reneges on a promise or when circumstances unexpectedly shift.