Hello Sush.

Upgradation means suppose you have filled up six options during councelling namely A B C D E F G H. Ok and let you are allotted choice F I mean college F then there are chances that after further few rounds your choice may get upgraded to D and afterwards to B. This is the system of choice upgradation this happen because many good ranker than you might have dropped this year.

Hope it helps.

Ok good luck..