Biomes

The Plains biome is the only biome where horses can spawn.

The grass looks different in different biomes. It won't be "fixed" because it's intended. If you really need your grass to be a certain color build in that biome. If you hate patchwork grass stop building on the edges of overlapping biomes.

"What about swamp, snow, and desert biomes?" Am living in giant swamp biome thats 10 blocks above the stronghold. World also has snow, is where I found pack of wolves. And 2 desert biomes the one nearest the swamp is beautiful m8. Huge ocean biome surrounding
mushroom biome island. Big underwater ravine in ocean aswell. Am also tired of seeing too many clay deposits around the swamp.

Also if you just want to update your world to its next seed generation biomes and build the new biomes yourself, just make a new map with the seed you used, find the coordinates of the new biomes, go back to your map, go to those coordinates, build your
new biome there, then do the hex edit, re-enter your seed into it, when the game loads up the save, it'll refresh biomes but not change your work.

When updates with new biomes or new things that generate in the world are added, there are normally some problems with the seeds, because they change. They added swamp and jungle biomes in previous updates, so sometimes you have problems with swampy, snowy
biomes and jungle biomes to spawn where they shouldn't, if this happend to you there is probably no way of getting the old map back, because the seed is different than it was...