NBA players are constantly moving from team to team in the modern NBA, and very often, we now see players leave one team to join their biggest rival. Most recently, Marcus Smart decided to sign with the LA Lakers, just two years removed from being regarded as the 'heart and soul' of the Boston Celtics, who happen to be the Lakers' biggest all-time rival.
Outside of Smart, other names that come to mind are Rajon Rondo, who spent nine years in Boston and won a Championship with the Celtics, but joined the Lakers four years later. Another player who did this was Dillon Brooks, as he left Memphis to play for their division rival, the Houston Rockets.