Why would they do that, though? Why change what the Remnants are? What the Seph copies are? Any of it?
I had to reeducate myself on the Remnants, and you're right. They aren't really the same thing.
But I still really dig the idea that people can become Sephiroth clones, just because it would fill in some blanks as to why there are JENOVA entities that are traveling all over the world, often further ahead of Avalanche. In my mind, the proper way to rewrite this is that there are a certain number of people abroad, including Cloud, each with bits of Sephiroth and Jenova embedded in their make up through Hojo experimenting on them after the Nibelhiem incident.
After the five year cycle when enough of Sephiroth has reformed in the North Crater to give him the strength to command those JENOVA cells, Sephiroth begins puppeting them. Cloud has enough sanity left over that all Sephiroth can do is tug at his mind, while weaker willed 'clones' starting with Number II act on the REUNION impulse, which Sephiroth co-opts to get his hands on the Black Materia.
From that points, he's been using his clones to string Cloud along, guiding him across the planet towards the Keystone and Black Materia, eliminating obstacles, such as freeing them from ShinRa Inc, and getting rid of the second Midgar Zolum, and testing Cloud/Co to make sure that they're strong enough to survive the journey. Even killing Aerith feels less like riding himself of an obstacle, and more just further baiting Cloud into pursuing Sephiroth. Otherwise the sensible thing would be to take the Black Materia and bury it away. But because Cloud is so irrational and revenge driven, he continues to go after Sephiroth.
This feels like a much more coherent way to write out the plot of the remake, since it gives Sephiroth a concrete and insidious reason to drag Cloud into the plotline that has a certain fridge horror element, and connects JENOVA more strongly into the plot by making her REUNION have a bit more agency than just being a McGuffin.
Mostly though, I love the idea that they could take this concept to the Nth degree, where at North Crater, they find out that under the cloaks, at least one of those 'clones' is a Silver Haired Cloud copy, bringing Cloud's identity crisis into overdrive.