The Replacing

'''The Replacing is a pre-Hardmode event that activates immediately after the following conditions are met. It replaces Corruption with Crimson and vice-versa.'''

Conditions

 * All Crimtane Ore/Corruption Ore has been mined.
 * All Crimstone/Ebonstone has been mined
 * All Shadewood/Ebonwood has been cut down

Results
When the above requirements are met, a message will appear  "'  The Forces of Evil have been aroused and have released Corruption/Crimson on your world.' "