Blizzard has confirmed that the new Myth upgrade track coming in Patch 10.1.5 will apply retroactively to gear which started at a base item level of 441 or higher!
Myth Upgrade Gear in Patch 10.1.5
Gear which drops at item level 441 or higher by default will now fall in the Myth track. Blizzard has announced that this is indeed retroactivate.
This includes gear from Mythic raid, Very Rare items from Heroic raid and Mythic+ Great Vault items.
