Updating from 1.5.1 to 1.6.0 seems to replace some BiomesOPlenty biomes on my pre-existing map with BetterEndForge biomes.

Example: My BOP Alp's Biome was turned into a Painted Mountains biome.

Reverting to 1.5.1 and rolling back the map resolved the problem. but that is not a permanent fix.

This is biome id shifting, try using https://www.curseforge.com/minecraft/mc-mods/biome-id-fixer