Hello,

If your seat doesn't get upgraded to a better branch in the next round, you will be Retaining the seat you got in the last round. In your case, even if you don't get a seat in CSE, your seat in ECE will be safe. It is a common thing and you should not worry about it. I know one of your senior (in NIT Silchar) who got Civil, then Mech directly in 7th round and finally settled in ECE after special rounds, now in the second year he is going to slide to CSE, so it a common practice and a seat you got once is never lost until you are upgraded to your preferred choice.

Hope it helps