Regarding to Kamijo, I don't think it can be said that he simply changed his vocal range and that's it. Rather, I think Kamijo changed his range because he learned how to sing properly. That is to say, he realized that songs like Lareine's (tenor range) are not suitable at all for his own natural range (clearly baritone). This is easy to prove to anyone who listens just to a single Lareine song. And in fact I think it was clear to Kamijo himself ; in fact, several songs from Lareine's repertoire were tuned down for live performances. Check "Metamorphose" as an example:
Album version (on C minor key)
Live version (on B minor key, and also an epic performance)
However, Kamijo might be still heard not so comfortable. Add to this the feminine and in-pain characterization that Kamijo tried to build for his on-stage persona. What you get is a voice that sounds like choked, it is clear that he is making a huge effort to be able to reach every high note. Also, I think that Kamijo smoked at that time, right? Much worse.
Either way, Kamijo had a very good vocal performance in Lareine. Not only because he always kept himself into the key (which in visual kei is like asking a lot lol), but because, despite all this I said, he managed to keep a flow of voice that was voluminous, and all that while charaterizing it in a very convincing and beautiful way.
Either way, fortunately he has decided to adjust to his natural range since Versailles began. His voice now sounds much more natural and versatile; he even can reach the tenor notes, which as I said are not natural for him, through some falsetto, which is actually heard very nice and dramatic.
I think the same can be said regarding Gackt. Check the following "Mizerable"versions:
Album version (on key of D flat minor)
Live version (on key of C minor)
Now there are other vocalists, that actually improved over the time, but in the beggining didn't try to lie to themselves proving that their range was higher than what their throats could actually offer without killing themselves into the process lol
Ruki have always had a beautiful voice and a decent technic, but I feel that songs like "Wakaremichi" or "Miseinen" sounds legitly good nowadays. Ruki now can manage to run trough the different notes and volumes very smoothly, while managing to bring out a strong flow of voice, even while delivering this gothic-inspired deep voice that is one of the trademarks of the 'visual kei' sound, but that makes singers of the scene really -like, REALLY- bad 90% of the times.
Album version (2002):
Live version (2012, and also another epic performance)
Also, Miku from Antic Cafe and Asagi from D can other great examples of vocal improving.