A switcheroo is a surprising change or reversal. If your mom replaced the cookies in a cookie jar with plums, your dad might say, "She pulled the old switcheroo!"
Most often, a switcheroo is a joke or trick — a magician might appear to change your ten dollar bill into a feather, and you could call this a switcheroo. Another kind of switcheroo is simply an abrupt change of direction, like a sudden switcheroo in a movie's plot that leaves you totally confused. Switcheroo was first used, in an informal and joking way, in 1933.