I opened a copy of my world (last saved in 1.17.1, created in 1.15.2 and played in all minor releases inbetween those two) in the latest snapshot and a chunk near my base and some surrounding blocks got replaced by what I believe is new 1.18 generation

I've created a world with the same seed (7596438851331474547) in 1.15.2, loaded it in 21w44a but no nearby chunk got reset

At least those surrounding blocks prove that smooth transition between old and new chunks works fine