Is it possible to add worldgen datapacks to an already existing world?

I've started a server a couple weeks ago, and i thought i had added Amplified Nether and Nullscape from the start, but the nether and the end are vanilla. Maybe i added them after generating the world, maybe not, but whatever happened, i didn't realise until a week in. Now, i've decided to reset the nether and the end so they can generate with the mods, but apparently just deleting DIM1 and DIM-1 doesn't do anything, they generate vanilla with the same seed. What is it I have to do?