My company uses a travel agent to book flights for its employees. Once they do that, I have no issues going through the website or app to change seats or use miles (or my own money) to pay for an upgrade. I still have some flight credits hanging around from 2020 (credit from a personal flight, not the TA). I would like to use it on an upcoming business trip that WAS booked through the TA, but I don't know how. Has anyone done this before?