I would think most of the recent Atmos upgrades from an existing 5.1. I hit surround late but hard. E.g. Have replaced several of the excellent King Crimson 5.1s with those available in Atmos (and will continue to do so). My wallet is on antidepressants these days
in consideration that 5.1 is seemingly less preferred now than Atmos, and there are a lot more out there to upgrade in the future.
Consider, say, the Moody Blues. Regular CD at first, then MFSL, and then 5.1s came along. Dump the CD, try to sell the
MFSL on Discogs for a profit, then prep for dumping the 5.1s if offered in Atmos (hell, the upmix on IN SEARCH OF THE LOST
CHORD was good! I'd chance Atmos upmixes).
Now multipy by all the classic bands you upgraded from CD - MSFL (maybe) - 5.1. Maybe I should unretire.