Minecraft - How do I find other ores once I have installed the other ores mod?

Hi,
I have just installed the other ores mod but i cant seem to find any of the new ores (such as silver) in my current world. However if i generate a new world i can find them straight away. So do these ores not generate in pre-existing worlds, only ones that you generate after installing the mod?
Thanks a lot guys for any help.

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