The mausoleum set WAS ORIGINALLY a cross which the Phantom emerged from behind, in London at least - with a pyramid of skulls and without opening gates or the pronounced Daae grave as featured in the current design but those elements (along with the significant amount of trailing ivy) were updated early on at Her Majesty's. Certainly within the first few years of opening. I would imagine the New York production would have reflected these changes in around the same time frame (if it didn't actually originate them in the first place). A few other updated design features were introduced to the New York show and then found their way back to London. Christine's Masquerade dress for example.
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