What are the implications for someone who bore false witness in an old case and now wishes to repent, fearing personal harm if they retract it, and believing their testimony did not affect the course of the case?

Bearing false witness is one of the gravest sins. Repentance from it requires desisting from the act, regretting it, and resolving never to return to it. If the false testimony did not cause harm, then repentance alone is sufficient without recourse to the court. However, if it resulted in harm, then one must go back to the court, recant the testimony, and seek forgiveness from the one against whom the false testimony was given.