You can do it manually with a bit of fiddling around in a world editor like MCEdit. MCEdit has an option in the chunk control tool to repopulate ores, althought last time I tried it it would crash if I used it on too many chunks at a time. Worst comes to worst you can either delete all chunks which haven't been built in so that when they regenerate they'll spawn new ores or, if your buildings aren't too spread out, you can copy your constructions, deleted the entire world, generate a new world with the same seed and then paste your buildings back in.
Make plenty of backups though, it's easy to mess this up or forget something.