Once you have successfully lodge application, then the bridging visa A should immediately come into effect when visa 600 expire. You can check status using VEVO the date after visa 600 expire to see if there is any bridging visa A in effect. If not, you can apply for bridging visa A separately https://immi.homeaffairs.gov.au/visas/getting-a-visa/visa-listing/bridging-visa-a-010#About
You can also apply for visa 600 again, then if you get acceptance for visa 884, just have to submit form to cancel the application for 600 before the new 600 get approve.
Advise is only from my speculation though, not expert.