ECNL-RL *is* the top level of NPL for NorCal. It has been for several seasons. It used to be called NPL Champions league. There is no choice for NPL teams to make in whether to move up or not. If they perform well enough in NPL1, the top few teams from each area compete in ECNL-RL for the next season. For U13, after that season, they come back to be called NPL again for the fall season U14, and in the spring once again the top few teams from each geography move to the ECNL-RL bracket for the spring. Once U15, if they make it to the ECNL-RL bracket, it is year round and they no longer come back to NPL automatically. However, there is continuous Pro/Rel for all of the age groups, where the bottom few teams of that RL bracket are at risk of being moved down to NPL1 for the next season, while the top teams of NPL1 take their place. If you are curious about how it actually works, just look here:
https://norcalpremier.com/competition/ecnl-regional-league/resource/ecnl-rl-overview/
Of course at any point teams/clubs could choose to opt out from these NorCal lower levels and try to join ECNL/GA directly, it's just not typically how it works. Many clubs have all of the above, and will have teams in these NorCal levels, as well as either an ECNL or MLS team.
This new announcement promises that it will be somewhat more complicated, and for 2025/2026 clubs now can put their teams in to this new "club-based" ECNL-RL bracket, or leave them in the existing ECNL-RL bracket. If most clubs favor the new bracket, that's where most others will follow, and the existing "team-based" ECNL-RL will not attract the same level of players/teams/clubs. It makes it less likely that a good team can work their way up without losing players to larger clubs. It was never highly likely, but it was possible from time to time. With MLS N now pushing MLS X2 and X2+ as yet another league for these 2nd/3rd tier teams to play in, it seems that all that can be predicted is that they won't all succeed equally. There are only a certain amount of kids, there is only a certain amount of money/time/resources, and we'll see what wins out over time.