I'm not a lawyer, taking advice from strangers on the internet is unwise, yadda, yadda, yadda...
1) It is legal. Theoretically the old body does not need to be retained. As a practical matter, the old body would be used to prove "prebanness" if it ever came to that.
2) The rule is that you cannot have enough components to create another mag. If you don't have components, you don't have to demil. However, demilling is easy and safe.
3 & 4) Don't know.
Personal opinion, with the prices on NIW pre-bans what they are today, it isn't worth the hassle to replace bodies.