Having seen the live show back in 93 (though honestly a lot of it was lost on 6th graders) and listened to the original cast recording enough to have memorized most of the songs, I am curious as to why they change up the words as they do. I'm not saying they're bad or anything, just different enough to notice. Is it just to make them flow better or something?
Shows evolve when they run as long as Phantom has. Sometimes it's the composer/lyricist wanting to make changes, sometimes a performer will tweak things and everyone likes it/likes their take on the character.
In Les Miserables for example, the Thenardiers calling Cosette "Colette" is something that evolved in the show after it was running for 10 years.