I am currently prosecuting Joe Defendant on 5 felony charges. It is a cluster...fart. Joe Defendant has sued his former defense attorney in JP court. Former defense attorney wants to call me as a witness on his behalf to "prove" that he earned his attorney's fees. Prosecution of Joe Defendant is still pending. I do not feel comfortable testifying as a witness in the civil suit - my question is, if I am served and testify, will that preclude me (and possibly my office) from prosecuting Joe Defendant? Former defense attorney informed me this morning that I am needed tomorrow. Great.

While not directly applicable, because you are not the attorney of record in the fee dispute case, 208 S.W.3d at 76 may provide a starting point. Of course, you also have no personal knowledge of what the attorney did in many respects.