Priority Date has the following effects:
1) NVC will not send Packet 3 if the date is not expected to be current in near future. If the dates retrogress after Packet 3 has been mailed, they will complete the process till they get it back from you/St. Louis. But will not forward it to the consulate.
2) Consulate will receive Packet 3 from NVC (after you have paid the fees and provided all the necessary documents/forms) only if the dates are expected to be current. However, if the dates retrogress after consulate receives the file (or is in the process of receiving), consulate will hold on to the file AND not schedule your interview till PD becomes current again. The reason for that is the consulate cannot get a visa number assigned to your case from State Dept. until PD is current. And hence there is no use of scheduling an interview.
3) Once your interview is scheduled, you are through. Your case has been assigned the required visa numbers. Every month State Dept issues visa bulletin, based on which consulates schedule interview. Between that time and your interview, if the dates retrogress, one is already through. the system is such that it won\'t retrogress for you.
i hope this helps.