Can a judge unilaterally change her ruling ?

Yes, but judges rarely do that. I would dare say you are incorrect it is likely that the prosecution made a motion.

To the best of my knowledge, a Judge cannot change a sentence once it has been passed upon by her. I believe they lose their jurisdiction once sentence has been imposed. However, also in my experience, many Judges do what they please and it will only be reversed if the new sentence is appealed by the People, since it appears from your question that the sentence was reduced after being initially imposed. I don't have a specific statute for you, however, you can check the CPL which is the Criminal Procedure Law. Good luck.