games – how to introduce new features to players

I have made some very big changes to my game and I obviously want all existing players (as well as new ones) to start using the new features I've implemented.

It's a MMORPG.

The updates bring a lot of radical differences to many parts of the game and alter some basic principles.

I wonder if I should:

  1. Let the elite / high level players continue where they left off and they will discover the new features while trying to play the new expansion.
  2. Force elders / high-level players – when they join – to complete the new tutorial (which all new players will do).

I'm afraid 1 is overwhelmed or 2 frustrated at not being able to get stuck in the game.

I think that in general, the best way to present the features to the players is to make them learn by "doing", like to progress in the world and learn skills, etc. necessary for the encounters that they have "naturally", in a certain way.

Is my intuition correct? Does this suggest that letting old experienced players continue where they stopped and get the updated content while they're playing is a better idea? (leaning towards 1?)

What should I do here? (And please, if you can provide sources / references, I am interested in the UX study)