1) Counterscam 2) Literary technique 3) Shift 4) Slangy reversal 5) Surprising variation 6) Switch 7) Switching 1) Swap 
A switcheroo is a sudden unexpected variation or reversal that is often associated with a joke (sometimes `the old switcheroo`). It is colloquially used in reference to an act of intentionally or unintentionally swapping two objects. As a comedic device, this was a favorite of Woody Allen; for a time, he used so many switcheroos that friends ref.....
Switcheroo is American slang for a surprising or unexpected change or variation.
[n] - a sudden unexpected switch
noun a sudden unexpected switch
an unexpected or sudden change or reversal in attitude, character, position, action, etc.