The only things I had to do on this was use TEdit to change the world to hardmode, and grab an alchemy table from another world for convenience, since the 1.3 items aren't included yet.
The wall of flesh refused to stay in the underworld and when killed, the corruption/hallow stripes appeared, but I never got the 'spirits of darkness and light released' message. Trying to smash an altar just resulted in damage, since the hardmode flag was never set. When saving and reloading the world, the corruption/hallow stripes were removed again.
Duke Fishron was challenging to fight, since the ocean biome is completely flooded to space. Had to take him on in his own element and do a lot of vertical moving using the turtle mount, neptune shell, and flipper potions. It can be drained, but it was fun doing it with it flooded. With him enraging when taken out of the ocean area, also have to be sure you don't get to the bottom as it is underground and turns to desert instead of ocean.
The cultists spawn in the starting planetoid, just had to dig out an area for them. The pillars themselves spawn on any planets they may be over, or at the surface/underground transition if they aren't over any.
Also a request for Atlantis mode. Could there be a solid row of some kind of blocks put just above the underworld layer to prevent all the water starting to evaporate slowly?