Blizzard is adding upgrade tracks for Mythic Item Level 441 gear in Patch 10.1.5.
Currently on live servers you cannot upgrade Mythic gear. However, that’s about to change in Fractures in Time when Mythic Gear will receive 3 upgrade ranks.
Rank 1 Upgrade: Item Level 441 > 443
Rank 2 Upgrade: Item Level 443 – 445
Rank 3 Upgrade: Item level 445-447
The change also applies retroactively to Great Vault loot / any loot obtained before the patch goes live, as per Reddit comments.