The easyest way:
* you earn one medal in a mission
* edit the CareerTrack.upc file in the save's folder
* replace all appearances of the id of the medal with the one you want to see in the game (one at a time)
* medal ids are found in the medals.upc file

Just to make sure:
* the medal textures are in the AwardsItems_1.dds to AwardsItems_4.dds files in Data\Menu\Data
* the new image should be in the same place as the old image (same texture file, same "crop")
* menu_1024_768.ini should not be touched