If the world's regions are already created under NORMAL type then subsequent regions will not regenerate in BIOMESOP type. I don't know if there's a way for the server to be aware of two simultaneous types. One NORMAL, one BIOMESOP.
It seems by experimenting that once the world is generated it's fixed to the type that was in server.properties regardless if you ADD the mod later.
Save off the world directory, Reset the server.properties world type, then ensure the mod is installed. Fire the server up and validate the biomes are within BOP variety.