What would happen if I used affiliate banners that were made for a certain affiliate program to promote a different one. Could I face any kind of legal action from the owners?
Yes. You can have some legal problems. Like any other images or material that is copyright, you need the permission to use the material of someone else. I suggest that you use your own banners. By using your own material, you will be legal and also nobody will confuse them with the existing owner.