how to introduce new features to players

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

It's an MMORPG.

The updates make a lot of drastic differences to lots of parts of the game, and change a few fundamentals.

I am wondering whether I should:

  1. Let old/high-level players continue from where they left off, and they will learn about the new features as they try to play the new expansion.
  2. Force the old/high-level players — when they rejoin — to complete the new tutorial (that all new players will do).

I am worried with 1 that maybe they will find it overwhelming, or with 2 that they will be frustrated they can't just get stuck into the game.

I feel like generally the best way to introduce players to features is to have them learn it by "doing", like to progress through the world and learn the skills etc. necessary for the encounters they have "naturally", in a way.

Is my intuition right? Does that suggest that letting the experienced older players continue from where they left off and pick up the updated content as they play is a better idea? (leaning towards 1?)

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