As other posters say, you'll definitely have chunks that suddenly change in elevation and biome type. You'll have to deal with it or restart unless you disable the biome mods altogether.
You have Ars Magicka in DW20 but not in Monster. You'll have to copy the mod from your DW20 install if you still want it. Check for other mods you might want to do the same with.
If you did want the worldgen @
rhn mentions, change the world type in server.properties to ATG (but you might also need to NBT edit your world files to specify ATG world type), which will generate the biomes of both of those Biome mods.
If you're feeling very enterprising, you might consider the time-intense task of deconflicting duplicate biomes: You'll find things like Savannah and Redwood forest in both biome mods, so you could pick your favorite and disable the other, or you could just pick one of the two biome mods).
Good luck!