Just enable this setting when booking your ticket to get a coach upgrade at no extra cost.

Auto Upgradation: If you have booked a train ticket and discover after the reservation chart is prepared that your ticket has been upgraded, do you have to pay a higher fare?

IRCTC Ticket Upgrade: When booking a train ticket, you typically choose a coach based on your budget and convenience; once confirmed, a seat is allocated in that specific coach. If you find during the journey that your travel class or coach has changed, you might wonder how this happened. This is actually the result of a specific setting selected during the booking process.

How does the coach change?