I didn't know this until today but when Nighthold launched there was a new quest added which allowed you to grind for essences in Nighthold to upgrade your iLevel 910 legendary to iLevel 940.
I didn't even realise legendaries had gone up to 940, but my rogue Pedigreechum already had Shivarran Symmetry at level 940 and Marcusmaximi's two legendaries are both 940.
I guess that's because I'm so slow at levelling.
If you do have an iLevel 910 legendary you can go to the blacksmiths in Dullaran and get the quest Touch of a Titan which send you to Nighthold to collect 50 Aman'Thul Essences - enjoy that grind!
Now I've heard that you'll soon be able to upgrade them further, although I can't currently find any evidence to back that up