Switching to Fast Track

Hello, I’ve applied for my first child’s passport. Documents still not logged in Liverpool despite delivering on the 10th. I’m getting too nervous as I fly 3rd May for a family wedding. Does anyone have experience of switching to Fast Track once they’ve already applied? I have another full birth certificate and HMPO just said I take a letter of withdrawal to the appointment, but the advisor seemed quite unsure so I wondered if anyone here had experience? Thanks