The surface height of the world was increased.

The surface height of the world was greatly increased. Opening my old world in 1.17 official release causes the world to adjusted height. The default ocean level before which is y-63 now was at y-127. Well some of my build was just fine is was adjusted and some were ruin just for example my build which at the height of around 240, suppose that the beta versions have y 320 build limits, now was been cut of at y 256. Furthermore while examining sa world the formerly bedrock which has limit at y 0, now it it's on y 64. To sum it up it seem that the old world had increased additional 64 y levels below the bedrock, I'm not pertaining to the y negative level. Analyze the screenshot below to figure it out.

And what's more, lot of my mobs, villagers, passive mobs etc, were gone, beacons reset, clouds obscuring vision, with the adjusted height, which is at height level around y128 129.