Here’s what you need to do:
- Be level 120.
- Unlock world quests, which will also unlock the 8.2 questline.
- Complete the beginning of the 8.2 questline, up to unlocking your first essence. It starts with the quest “The Wolf’s Offensive” (Alliance) or “The Warchief’s Order” (Horde). You only need to complete it up to the quest “Harnessing the Power”. After this, when you exit the Chamber of Heart, you will automatically start the 8.3 questline.
- Complete the very beginning of the 8.3 questline, from the auto-accepted quest “An Unwelcome Advisor” (Alliance)/“Return of the Black Prince” (Horde) up to and including “Network Diagnostics”.
- Completing the quest “Network Diagnostics” (helping MOTHER scan the titan facilities of Azeroth) will unlock both the new Uldum and the new Vale. You will be able to access both zones, though you won’t be able to actually do content there until you progress further in the questline.
Zidormi is available in both Uldum and the Vale to switch to their old versions, though the Zidormi icon in the Vale didn’t show up for me until I entered the new Vale for the first time.