They can be upgraded normally even after they obsolete.
Thanks for the info, but it was not really the point of my question, which I can resume in two points:
Firstly, in term of strengh, the Obsidian Neanderthal warriors have the same as their metal upgraded version, which make them useful even if you happen to not have metal in your territory. The notable exception was, as I mentionned before, their version of the Obsidian Swordman, which have one strenght over his regular version making it having the same strenght as the first regular swordman. In older versions, it only could upgraded once you get iron, which was well after the others Neanderthal warriors, which you upgrade not by obligation, but rather by mere choice: to obtain the, latter and stronger, Swordman units. But now, as they become obsolete at Sedentary Life, they are replaced by their normal Obsidian Swordman version, which I think, was not really an upgrade at all.
And secondly, why do they have to become obsolete in the first place? I mean, if you start as Wrud, by the end of Prehistory Era, you have a couple of cities in your control (in my case, normally 10), all giving you the Neanderthal culture bonus, so making the Neanderthal units all becoming all obsolete, in only one turn (and that include the Neanderthal Mammoth Rider, the Neanderthal Workboat, the Neanderthal Wise Woman, the Neanderthal Worker, and the Neanderthal War Dog), don't really represent the situation in which your empire are. So, why having changing them from the older versions (as choices you have to made), in which you could had, even, build Classical Neanderthal Workboatm, as you managed to still well alive far into the timeline?