Usually, newly added things that generate in world will be generated in newly generated terrain. Just like in vanilla, when mojang adds something to the game and you update your world to the new version of minecraft.
That said, some mods provide the config option to generate in existing terrain. This option in called "retrogen." Retrogen will re-populate all the existing terrain chunks with the ore/plant/tree/structure whatever that that mods retrogen option is programmed to cover. However, it may cause problems.
It may retrogen to much if the option isn't fine tuned enough, or worse, doesn't allow fine tuning. It will also generate ores in places such as the previously cleaned up walls of your basement, if you made them out of stone. Or it can generate structures or trees overlapping with thing you've build, thus breaking them.
If you use mods such as CoFH world to generate an ore, instead of the mod the ore originates from, then the retrogen comes from that mod. In the case of CoFH World, this is a good thing because CoFH word's retrogen can be fine tuned just like everything else in that mod.