Save it in the program then load up your world. If you want to do this, first exit the world, then load the world in Tedit (I will explain how to in just a second), then under World properties simply check the box where it says hardmode. If you want, in Tedit, you can actually set your world to become hardmode without having to defeat any of the bosses. I would strongly suggest having one corrupted/hardmode world separate, because you need certain hardmode mobs and ores that you will not be able to obtain once you cleanse your world. (you can do this on existing worlds also, it doesn't matter).

Make any type of world any size on any difficulty, and make sure to load into the world. Now before we mess with the application, first we need to get the world set up. (Most likely the latest version will work) Once you have done that, extract the files and open up the Application from inside the folder you just extracted. Make sure to download the correct version corresponding to the correct version of Terraria. If that doesn't work, then just look up Tedit downloads. To start off, you need to download some software called Tedit. Some say these add a nice challenge to the game, but if you are like me and you just want to sit back and relax, then this is for you.

This process will COMPLETELY remove the Corruption/Crimson without doing it manually in-game. Hello Everyone! while this solution is really easy, it seems as though I haven't found this anywhere else.

Terraria thorium map viewer