Blizzard has released a massive developer blog going over all the gear upgrade system changes coming in Patch 10.2 and their thoughts on why they did some of these things!
They've confirmed that the conversion rate for the Trading Up crests is 6-to-1, confirming the speculated 90 Lower Crests for 15 Higher Crests rate.
In Guardians of the Dream, we’re keeping the system in place but will be making a suite of changes—many based-on player feedback—to make this system both more sustainable and more player friendly.
The Myth track is three upgrade levels long. We’re lengthening each of these tracks by one upgrade level.
In Dragonflight Season 3, the Hero track will be six upgrade levels long, letting players upgrade their gear up to Hero 6/6. In Dragonflight Season 2 item level (ilvl) terms, this would be an increase to ilvl 444.
Read more on wowhead.com