There are 4 major sports leagues in North America: NFL, MLB, NBA, and NHL. The teams, also known as the franchises, in the league don't change. The only exceptions are when a team moves to a different city (still the same franchise) and when the league expands by adding more teams.
This is very different to European football leagues where promotion and relegation is practiced. A small number of teams that finish last in a season are relegated to the lower league, and the top teams of the lower league are promoted to replace them.
Has there been a time where the North American major leagues practiced promotion and relegation?