No, they are not interchangeable. A guardian is someone who protects someone or something in a more abstract or moral way than a guard, who protects something physically. So a legal guardian can be someone who looks after a child in the place of a parent. A religious figure might be described as a guardian of their faith. A guard could operate in the context of a prison or a castle. A guard could also be an object, like a mouth guard.

The word guard can actually be used as a noun or a verb. If it is used as a noun, guard means someone who protects if it is used as a verb it means to protect. On the other hand, guardian is a noun which means "someone who is legally responsible for a person."