Witness vs. Defendant

Can a witness in a criminal case actually end up becoming the defendant?

Sure can.

Is this being handled under the same proceeding or a brand new one? If someone gets called in specifically to give testimony as a witness, will they stay categorized as a witness during those discussions, or could the court pivot and issue a different ruling entirely? Also, what’s the deal with an attorney's witness—does anyone actually need to formally advise them of their status? Just a mountain of questions and uncertainty here. I'd love to hear from any legal experts out there (or even just a friendly voice)

Honestly, I don't care if they try to charge him as a co-defendant or just bring him in on a separate count. A witness doesn't even need an attorney.
Then there's this side of it too...
Čanak and section 246 of the FBI code...

A witness isn't actually required to answer specific questions if doing so would likely leave them or a close relative facing serious embarrassment, major financial loss, or criminal prosecution. The court is supposed to walk the witness through those rights.

p.p.