Texture Replacement Tool
This is a handy little tool for use when you want to replace all instances of one texture with another texture. Open with:
UnrealEd Main Menu → Tools → Replace Textures
Select the texture you want to replace in the Texture browser and click set in Field 1. You should see that texture appear.
Now Select the new texture you want to replace the old one and click set in field 2.
Now just hit Replace. Tada
Warning: There's a bug in the texture replacement tool that will prevent you from opening it again once you have closed it until you restart UnrealEd. So, if you want to replace multiple textures, just don't close it after each texture.