I don't like Hell, can I go somewhere else?
My server runs 1.8 - but the 1.9 update will introduce some nice new Nether features.
So is it possible to generate a new nether without generating a new world?
As far as I am aware, the nether is just another directory in your worlds directory called
DIM-1. From what I understand, removing that subdirectory will remove your nether and force Minecraft to generate a new one upon visiting it (using the current terrain generator - this means you may get different Nethers across different versions of Minecraft if the terrain generator has changed between them).
But please do make a backup :)
Make sure Minecraft isn't running in the background while trying to make the changes to your DIM-1, because it won't work.