Easy Retrogen is a utility mod that allows mods with world generation to perform re-injection of their content into existing generated chunks, and tracks which ones have been retrogenerated.
This allows you to add a new mod to your world and have it's content retrogenerated into existing chunks. Example: You created a world and post-installed IndustrialCraft 2, EasyRetrogen will regenerate all ores from IC2 in your already existing world.
It works with the IWorldGenerator based system from FML used by most mods to inject simple content into world generation.
listretrogens - lists all IWorldGenerator classes added to Easy Retrogen.
To configure this mod you need to figure out which classes implement IWorldGenerator from the mod(s) you wish to regenerate.
As an example, slimeknights.tconstruct.world.worldgen.SlimeIslandGenerator is the worldgenerator for slime islands in Tinkers Construct.
Tip: you should, if the mod is open source, take a look at it's GitHub repository and figure out the name of the class you wanna add.
This mod is a fork of cpw's simpleretrogen mod which can be found here.