I just created a world in 1.0.0 and all previously generated biomes converted into other biomes, for example, a swamp converted into snowy tundra or desert converted to forest.
Relates to MC-31205
I didn't test yet but I think it will 99.98% certainly happen if I convert an old world from the first version that had prooper biomes - alpha v1.2.0.
Does this happen when you convert the world from 1.0.0 to 1.14.4 as well? Or only in 1.15?