Upgrading Legendaries
You can upgrade existing legendaries to ilvl 1000 after Antorus opens via  Awoken Titan Essence. The Patch 7.3 Q&A stated that upgrading your legendary to ilvl 1000 should work much in the same way as past upgrades. As such it will work similarly to past upgrade quests:
Collect 50  Wakening Essence to create  Awoken Titan Essence, used to upgrade one legendary to ilvl 1000. You will need to repeat the process for additional legendary items.
Note: This item works on any legendary below ilvl 1000, including old 910 or 940 legendaries.
 Wakening Essence should be obtainable from:
Weekly Mythic+ Chest
Antorus, the Burning Throne bosses
Lootable once per week per boss, regardless of the difficulty
Emissary caches
First daily Legion heroic dungeon
Weekly PvP rewards
Awarded on the first 3 Rated Battlegrounds wins and 10 Rated Arena wins.
 Cursed Shard starts (Quest #49474) which rewards 10  Wakening Essence. Currently unknown source.