But what about multiplayer? 6-7 players in small world can dig up almost everything...
I don't like idea about auto-converting ores either. I like pre-hardmode ores, and usually use Copper and Iron even in Hardmode (copper roofs are great, and Minecart Track needs Iron).
But, for example, you can craft from
Miracle Fuel and
Pixie Dust something like
Purification Powder (lets name it
Hallow Powder and
Cyber Catalyst). You can spread this new powder on pre-hardmod ore blocks, which cause their conversion into hallowed/cybernetic ore, like PurPowder converts corrupted stone blocks into normal ones.