FlatRegionFunction replace = new BiomeReplace(editSession, target);
if (mask2d != null) {
replace = new FlatRegionMaskingFilter(mask2d, replace);
}
FlatRegionVisitor visitor = new FlatRegionVisitor(Regions.asFlatRegion(region), replace);
Operations.completeLegacy(visitor);
player.print("Biomes were changed in " + visitor.getAffected() + " columns. You may have to rejoin your game (or close and reopen your world) to see a change.");
}