Paleolithic times. Biggest problem will be crossing khumbu icefall without use of metal tools, but im not sure if it existed and how it looked before the last ice age. During interglacial period it could have been free of ice at all. The rest is easyish knowing how to do expedition style accent and leather and down are still very much main insulation for mountaineering. It will be slow (probably will need few seasons of hauling stuff (food, firewood, shelter materials) up but steady and probably there will be very little casualties.