Let's get the obvious out of the way up front, shall we? There is not currently, nor has there ever been, a full, traditional system of on-field-merit-based promotion and relegation in any American pro soccer endeavor that I'm aware of[1].

With that said, the essence of promotion and relegation, the ability to move between levels of a nebulous pyramid, does exist, and has for more than a decade[2].

I've already examined the NFL and NHL, so now it's time for professional basketball.
What if the NBA had implemented a system of promotion and relegation? There was no obvious point to start like with the NFL and NHL, so I settled on the 1967-68 season. That's when the league was expanded to 12 team, the same starting number I used for the NHL, and it also means that all three of these posts started right around the same time.

Last week, I looked at what would've happened if the NFL had promotion and relegation. Today, I look at the NHL using the same rules.
Like the NFL, there's a clear point in which implementing it would've made sense. That's the 1967-68 expansion which saw the size of the league double, from the long-standing six teams to twelve.

The year is 1970. The Kansas City Chiefs have just defeated the Minnesota Vikings in Super Bowl IV, evening the score at two wins each for the NFL and the AFL. It's now time for the merger between the two leagues to take place. But instead of the Baltimore Colts, Cleveland Browns, and Pittsburgh Steelers jumping to the new American Football Conference and making one 26 team league, the owners instead decide to take a cue from European soccer and institute pro/rel in a major American sport for the first time.