While there have been plenty of movie to stage musical to movie musical adaptations (The upcoming The Color Purple and Mean Girls are two), this would be the first (I think?) movie musical to stage musical to movie musical adaptation.
The current stage musical maintains a lot of the songs used in the original movie (like "Tango Roxanne" and the only original song, "Come What May"), but also adds (snippets of) many more. Luhrmann's statement that "So I can see in 20 years you recalibrating it again with new music." seems to speak to something else again - the idea of doing yet another take on Moulin Rouge (whether on stage or on film) with music drawn from the bigger pool available in another generation or so. Sort of like Disney's idea of revising Fantasia every so often.