Hello,my name is Judith. Thank you for this opportunity to answer your questions.
Here are direct answers to your question:
Do any laws require the school to withdraw the petition in this case?
There is no law that requires you to employer to withdraw the petition. They may leave the petition in place.
Does this change will affect my green card?
Yes. When your priority date is current you will need an offer of employment for a same or similar occupational job classification which you do not have as you are no longer a tenured professor and your new job is as an assistant professor.
If it does, how should I do to keep the application valid?
The best way for you to keep your adjustment of status on track is to find a tenured track professorship with a new university. You may port your approved I-140 to a new employer offering you the same or similar occupational classification since your I-140 is approved and has been pending over 180 days.
That is the best and only way to keep your adjustment alive. You do not have to work for that new employer until your adjustment is approved. Therefore, you can begin your job search and attempt to start the new tenured track job while the adjustment is pending or upon approval.
I hope this has answered your questions. You can ask me further follow-ups. Be sure to address the follow-up to JUDITH since a previous expert has put out an Information request to you. I saw you had requested me and wanted to provide you with an answer.