It happened before. In 2010, Paul Pierce and his Celtics teammates defeated Lebron James' Cavaliers in the Eastern Conference Semi-Finals, and shortly thereafter The King was off to the Miami Heat.
According to CbsSports Jack Maloney both Lebron and Coach Tyronn Lue wanted Cleveland to trade for the Clippers' De'Andre Jordan prior to the trade deadline. That never happened, and the Cavaliers are now down two-games-to-none to Boston in the Eastern Conference Finals.
The trades for George Hill, Larry Nance, Jordan Clarkson and Rodney Hood brought in some younger role players, but all have been relatively ineffective versus Boston's tough defense.