If someone has already testified to a certain "fact", then recants that "fact", does that not make the previous testimony perjury? Otherwise, why bother to take an oath to tell the truth before testifying if you can change your story willy-nilly? Like I said, I'm not a lawyer (obviously), but common sense would tell me that it does.If "my own" witness changes their story from what they previously told me or testified to, absolutely. This is all done out in the open on the witness stand. It's not "witness tampering".
As for "my own" witness, many times a witness claims to see something or know something. If I want to get the information they have, I call them. They are technically "my witness", but that doesn't mean I have any special control over them.