Here’s a general explanation of how these things work.
Chromie Time is less of a phase and more of a shard. (Phasing is what controls the current state of the world, sharding controls which zone server instance you are in and what players you see.) So, turning Chromie Time on for any expansion will put you in the Chromie Time shard when you leave Orgrimmar or Stormwind and everything will scale up to 70. The only thing the expansion you choose affects are
- Which dungeons you see in the dungeon finder.
- It will give some priority to that expansion’s quests on the Warchief’s Command/Hero’s Call boards.
- It will give you a breadcrumb quest that will put you in the relevant phase for that quest — in theory at least, sometimes it can be buggy. (So, if you choose Shadowlands, you’ll get the quest “A Chilling Summons” and will be able to see Morgrain and friends in Stormwind and Orgrimmar.)
Beyond #3, Chromie Time doesn’t have much of an affect on phasing. (As Nobully pointed out, the NPC Zidormi controls that in zones with multiple versions.) Phasing is controlled by what quests you’ve done and what quests you are eligible for.
It’s that second one that causes all sorts of fun (and it probably what you’re hearing about when it comes to certain levels affecting phasing) as certain quests become available at certain levels. A quest that has nothing to do with what you are currently working on can suddenly become something your character has access to, and that’s enough to change the phasing. (50 used to be a big one for some zones as that was when BfA end game content came on line, and BfA had end game content in several older zones.)
By default, the game will generally favor the newer phase of a zone unless you do something to specifically swap it back, such as talking to Zidormi. I do not know what has changed with The War Within prepatch, so I don’t know what levels create the fun anymore. (And Blizzard fiddles with the phasing priority all the time behind the scenes, so it’s not to unusual to find things working differently for you than they did for someone a year ago.)