This has been done (relatively) long ago. It's called metamorphism - the entire code of the virus is continuously changed. They're incredibly complex pieces of

This has been done (relatively) long ago. It's called metamorphism - the entire code of the virus is continuously changed. They're incredibly complex pieces of code; in at least one case, 95% of the code was devoted to the metamorphic engine. Note the difference with polymorphism, where only the decryption routine[s] change. Two notable viruses have been Zmist and Simile.