Short answer: IRCTC as a booking platform doesn't have a "request a seat change" feature once your ticket is confirmed. But that's not the same as saying nothing can be done — the confusion usually comes from mixing up three separate systems: the booking platform, the chart/allocation process, and on-board TTE discretion.
What IRCTC (the booking system) can and can't do
Once you've booked, IRCTC's own interface lets you cancel, modify boarding point (within certain windows), or check PNR/seat status — but it has no mechanism for swapping a confirmed berth for a different one at your request. Any berth movement you see happening automatically (say, moving from RAC to Confirmed) is the reservation system processing cancellations and the waitlist, not a "request."
What actually determines your final seat
Your seat is finalized in stages: at booking (provisional, especially if RAC or waitlisted), at chart preparation (the first chart is typically prepared a few hours before departure — timing depends on the train and route; this is when RAC/waitlisted passengers most often see final confirmation, and a final chart is prepared shortly before departure), and on board (subject to whatever the TTE's records show and their discretion for any changes).
So what can a passenger actually do?
Wait and check — most "upgrades" happen automatically through the chart process, not through a request (here's what actually shifts after you book). Ask the TTE on board, understanding it's discretionary, not a right — which is a very different path from a direct swap between passengers. Arrange a direct, mutual swap with another passenger in your coach — this is the one lever fully within your control, and it's completely normal practice as long as it's consensual and the TTE is aware.
CoSeat exists for that third option: instead of hoping to run into a willing co-passenger, you verify your PNR and get matched with someone on your train who wants what you have and has what you want.