Steps to Reproduce:
Create a world in 1.17 with a datapack installed that affects world height. Then, reopen it on 1.18.

Expected Behavior:
Biomes stay as normal, and removed biome variants turn into the normal version (badlands plateau becomes normal badlands, giant_tree_taiga_hills becomes normal taiga).

Actual Behavior:
The blocks themselves will stay the same but the biomes of ALL loaded chunks will become plains biomes. This is true regardless of whether they are biomes that 1.18 removed (modified_badlands_plateau, giant_tree_taiga_hills, etc) or biomes that 1.18 did not remove (regular desert, regular savanna, etc).