Immigration status after leaving job when H1b approved (COS) before October 1st

I am currently in STEM-OPT and my H1B case (COS) approved last week. I recently got a job offer who are willing to start the GC process (ROW quote). If I switch jobs, what will be status be, STEM-OPT or H1B? Do I need to ask the current employer to withdraw the H1b petition or will it be automatically canceled?