So it looks like this is never going to get fixed. If you'd like to fix your own world, you can do it this way:
1) Download ClimateChanger from https://github.com/rutgerkok/ClimateChanger
2) Point it to your world's region folder, and change region 5 to 30. This will convert all Taiga biomes to the new Cold Taiga biome.
3) Now do it again and change 19 to 30. This will change all Taiga Hills biomes to the Cold Taiga biome.
Done!
As you explore new chunks, you'll still be able to find the new Taiga biomes, both the snowy and non-snowy varieties, but your old Taiga biomes will remain the winter wonderlands they've always been.
So it looks like this is never going to get fixed. If you'd like to fix your own world, you can do it this way:
1) Download ClimateChanger from https://github.com/rutgerkok/ClimateChanger
2) Point it to your world's region folder, and change region 5 to 30. This will convert all Taiga biomes to the new Cold Taiga biome.
3) Now do it again and change 19 to 30. This will change all Taiga Hills biomes to the Cold Taiga biome.
Done!
As you explore new chunks, you'll still be able to find the new Taiga biomes, both the snowy and non-snowy varieties, but your old Taiga biomes will remain the winter wonderlands they've always been.