In Terraria, when you create a world, there are only certain materials that generate in that world, specifically ores. Platinum and gold are examples. Can you change the ore a world generates?
You can not change the ore a world is set to generate, however, I have come up with two solutions that do me quite well.
Using the Extractinator: Farming Silt and Slush
With an Extractinator, you can convert Silt and Slush into any number of useful materials. While completely random, you will commonly receive coins, jewels and ore. Before creating my off-world mines, this was a stable source for silver, gold and platinum. I also found it to be the quickest way to earn gold. I understand I got very lucky in finding one so early in the game, with ice chests having only a 5% chance to contain one, however until you're as lucky, just make sure you're putting all that silt and slush away, and don't destroy it.
Generate Mining Worlds
Once I pulled my character up a bit to take on the underworld, I got myself kitted out with Mining Gear. The helmet, alone, gives you a free light source. With the other two pieces, you gain a 30% increase to mining speed.
I then generated a new world, and proceeded to turn it into an "off-world mine". Dig tunnels through the world, until you find the materials you are after. Dig out a bit, to reach lower levels that contain higher-value ores, and dig out tunnels in a checkered pattern - it just makes it easier to navigate back to the farthest reaches. If you take a spare bed with you, you often find the odd underground house you can use to set up a safe re-spawn.
Off-world mines are still limited to the ore they generate; however, they can also be a very good source of silt and slush, to which I direct you back to heading one.
Short answer: No
The world will be generated with only one of these ores (chosen at random) and once generated, you cannot change the world other than through standard gameplay.
Without using a world editor, a world cannot contain natural ores of a type it didn't generate with. Though people could still bring ore mined from another world in and place it.
If you specifically want a world with one type rather than the other, your best option is simply to generate a few worlds and wait until you get one with the ores you want.