I think I've caught the AI in the act of altering levels as I approach them.
In one of the episodes that I beat yesterday, I remember there being one or two Legendary levels. As I approached the end, I thought to myself, "Wait a minute. I don't remember seeing FOUR of them! Weren't there only one or two?" Before I closed Candy Crush Saga for the night, I took a screenshot of the next episode in case I was just being forgetful again.
Episode 445: levels 6801-6815
Levels 6801-6807 were all unrated.
As of this screenshot:
6808[SH] - 6809 - 6810[SH] - 6811 - 6812[SH] - 6813[L] - 6814[SH] - 6815[L]
(H = Hard, SH = Super Hard, N = Nightmare, and L = Legendary)
When I passed level 6804, I noticed that the difficulties changed:
6808[SH] - 6809 - 6810[SH] - 6811 - 6812[H] - 6813[N] - 6814[SH] - 6815[L]
Levels 6812 and 6813 had been downgraded by a tier.
Then, I passed level 6805, and the difficulties changed again:
6808[SH] - 6809 - 6810[SH] - 6811 - 6812[H] - 6813[SH] - 6814[SH] - 6815[L]
Level 6813 had been downgraded again! It used to be Legendary, but now it's only Super Hard.
I passed level 6806, and the difficulties changed for the final time:
6808[SH] - 6809 - 6810[SH] - 6811 - 6812[H] - 6813[SH] - 6814[L] - 6815[L]
Level 6814 had been upgraded straight to Legendary.
No more changes were made to the level difficulties. I managed to get through this episode with barely any trouble, only needing to use a lollipop hammer on level 6814. Ads came in handy.
・・・
This episode was perfect for observing difficulty changes. The difficulty ratings followed a very obvious pattern: every other level starting at 6808 was Super Hard, and the last two non-Super Hard levels were Legendary. Changes to the difficulties were immediately noticeable. So yes, the game likes to mess with level difficulties as you play!
In case you'd like to compare levels, here are my versions of levels 6812, 6813, and 6814:
Any differences between players' levels will surely be interesting to see.
