My understanding is, as they got more experience with the game, and fine tuned the requirements, existing spots were left alone, even they broke the new requirements. But, if anything interesting happens, then the requirements would be revisited any applied.
Now, how they might apply that is probably unknown. Maybe they select the oldest spot. Maybe the one with the most interactions. Maybe they use a random number generator. Any maybe they even change what approach they use for selection over time (again, as they gain more experience).