Yes because the status is seen as on the date of appointment. Therefore PQR & CO. can continue to be the auditor.
I agree with abhimanyu. in common partner situation you have to see status at the of appointment. so if no common partner situation as on date of appointment, then is absolutely fine, dosnt matter if common partner situation arises after appointment.
there is hidden assumption that partner has not left old firm.
if partner leaves old firm and joins new firm or makes new firm then such firm is not eligible from date of joining irrespective of status on joining date