rinnegan as an "upgrade" from sharingan makes no sense to me, and I'm tempted to say that it is a mistranslation.
doesn't upgrade suggest to you that all the original abilities of the sharingan are present in the rinnegan? But whenever madara uses a sharingan ability, he's showing sharingan eyes, and whenever he uses a rinnegan ability, he's showing rinnegan eyes. Why would he need to do that? And for that matter, rinnegan is pretty sweet, but there are abilities that sharingan has that would be occationally useful, but Pain never uses them. Is he locked into the rinnegan because tobi gave him eyes in rinnegan mode, and its stuck that way (like kakashi's sharingan seems to be)?
It seems more sane to me that the rinnegan isn't an upgraded form of the sharingan in the same way that the mangekyo is an upgrade from the standard sharingan. Rather, Madara got his hands on a pair of rinnegan, and figured out a way to swap the two out in battle, perhaps using something like kakashi's kamui. You could say that he "unlocked the eyes" by figuring out a way to use them as well as his sharingan, rather than instead of his sharingan, since, being familiar with the use of sharingan he preferred to keep his sharingan rather than swap out to the rinnegan.
doesn't upgrade suggest to you that all the original abilities of the sharingan are present in the rinnegan? But whenever madara uses a sharingan ability, he's showing sharingan eyes, and whenever he uses a rinnegan ability, he's showing rinnegan eyes. Why would he need to do that? And for that matter, rinnegan is pretty sweet, but there are abilities that sharingan has that would be occationally useful, but Pain never uses them. Is he locked into the rinnegan because tobi gave him eyes in rinnegan mode, and its stuck that way (like kakashi's sharingan seems to be)?
It seems more sane to me that the rinnegan isn't an upgraded form of the sharingan in the same way that the mangekyo is an upgrade from the standard sharingan. Rather, Madara got his hands on a pair of rinnegan, and figured out a way to swap the two out in battle, perhaps using something like kakashi's kamui. You could say that he "unlocked the eyes" by figuring out a way to use them as well as his sharingan, rather than instead of his sharingan, since, being familiar with the use of sharingan he preferred to keep his sharingan rather than swap out to the rinnegan.