As always, officially NGUI supports only the latest version of each major Unity release -- meaning 4.7, 5.6 and 2017. Unity changed some shader defines, forcing shaders to auto-upgrade in Unity 5.5-5.6, making all projects opened with that version of Unity incompatible with the previous. It's not possible to change it back, because as soon as you tab back to Unity, it force-upgrades it again.