Lets say in Terraria you were to mine out every ore block in the world, then break two demon altars in hard mode, put all back down in one large vein would it all change to titanium/adamantium? Or does the game keep track of which ores have been mined and which ones were placed?
Hardmode ore generation doesn't work that way, it doesn't replace existing ore veins with the new hardmode ore deposits. Hardmode ore can definitely spawn in some placed blocks (I'm unsure about if it can spawn in Bricks, but player placed Stone, Ash, etc... work fine).
The best way to think of the ore generation is that the new veins replace existing blocks (i.e. it cannot create new blocks where there are none), rather than existing types of blocks. If you were to dig up the entire world and just pop down one large deposit of ore, you'd get very little hardmode ore out of it, as there wouldn't be many places for that ore to spawn.
I actually tested this out on an old world to be certain (I deleted all terrain in a hardmode world and then created a block about 128x128 of Gold Ore, stuck a Demon Altar on top of it then went in game and smashed it - almost no ore spawned), you'd end up with a tiny amount of hardmore ore, if any at all.
Edit: Actually in my test world no ore spawned at all. Seems as though it doesn't overwrite existing ores, just 'junk' blocks like stone.
Further edit: Ore does spawn when I use Ash blocks as a base instead of an ore, however it lags horribly while (presumably) it's trying to find valid locations to spawn the ores, and you don't get much of it when it does spawn.