With level-type=RTG the server just used normal terrain generation. This is on a pre-existing world.

pre existing RTG world? or was it vanilla originally

I'm pretty sure the served world stores its level type somewhere and you have to change it there for an existing world. Level-type in the main directory is for new worlds.

I really wouldn't advise changing worldtype for an existing world. You'll get really nasty chunk walls.

If you do decide to change the level type, you can do that easily with NBTExplorer.
If not, what I'd do is use MCEdit to copy my base or whatever else I care about and place them in a fresh RTG world.

I'm pretty sure the served world stores its level type somewhere and you have to change it there for an existing world. Level-type in the main directory is for new worlds.

This is correct, it's stored in level.dat. To change it you will need NBTExplorer (or another NBT editor) and you need to set level.dat -> Data -> generatorName to 'RTG'.

But as Zeno said, this is highly ill-advised.

@parcel31u I'm closing this, but feel free to comment on this issue if you need to.