When you add new mods after the world has generated (this also applies to the base game being unmodified and you just updating to the next version on an existing world) the new mod added content will appear in unexplored areas of the world not the ones you already have due to the way the game reloads explored areas of the world it doesn't regenerate (as far as I know at least). Making a new world is a good idea but if you want to use the same one just make sure to look in areas you haven't.
For ores/items/blocks being added/removed (just encase you decide to later) it's not too bad if say you don't want to use the mod later, but for biomes when they make up a lot of the world the newly added blocks, plants and more, it can be awkward and the game may crash if it can't handle it as it can't adapt to the changes when trying to load blocks that don't exist or various other reasons. it depends on how much the mod really effected the game though so it might not crash always. So be careful when using world generation mods, but you usually shouldn't have any major issues with ore related ones or ones that add lots of blocks (from my experience at least).