What is the best way to handle cases whose judgments have been made void, after the defendants have already begun fulfilling their sentences? A judgment in absentia? Nunc pro tunc orders? Dismissing the cases without prejudice and then re-filing them and having a judge enter an order giving credence to those orders entered in the previous cause numbers?

More facts are needed to understand your question. Why are the judgments void?

For reasons I am not able to elaborate on, the judge decided to void the judgments.

Did the judge grant a motion for new trial, issue a verbal order that they are void or what? Does the judge simply want the judgment to be in a different format or does he/she want a differenct sentence? What is the problem with the previous judgment?

What authority does a judge have to simply "decide" the judgments are void? Don't they have to meet one of legal requirements that make a judgement void?