When upgrading a world from 1.17.1 all biomes are completely changed, making the chunks outside of all 4 places we built have a completely different biome, is this intended? It will be very hard to upgrade to 1.18 without using an external program such as MCA selector to mark all chunks in quite a far distance.
Examples:
Plains with a snowy mountain close by become a snowy taiga.
Forest/plains/mountains is replaced with a giant frozen ocean with ice spikes and icebergs.
Savannah on the other side of a warm ocean from above plains is in the same frozen ocean.
This is working as intended. 1.18 introduces completely new world generation that has been under development for a few years.
To mitigate this, 1.18 also introduces terrain and biome blending at the borders of old and new terrain.