Re: You guys noticed....
I'll try to make it clear.
Originally Posted by Tsujan9484
Your eyes base mode is the version you're implanted with. If you get a set of Rinnegan as implants, you can't go back.
But, if you get a Sharingan and upgrade it to MS yourself, you can switch between the two at will.
Think of Kakashi, his left eyes base is still a Sharingan but he can switch between the normal Sharingan and MS because he activated MS himself.
Nagato was directly implanted with the Rinnegan. Madara somehow unlocked the Rinnegan following the path of Sharingan -> MS -> EMS -> Rinnegan. We can be sure of this due to the following manga pages.
The difference is that while Nagato was implanted with the Rinnegan, Madara unlocked it, he was the original master of said eyes.
That's the reason he could switch between them at will.