hi! once you are allotted a college via the neet councelling in the first round, you can opt for up-gradation in the second round. if you do get a seat and accept it, your documents will be transferred and fees will be adjusted accordingly. then you will have to report to the college allotted in the second round. But if you accept the seat and dont report to the college allotted in the second round, you will lose both the seats.

But if you are eligible for ugradation and somehow want to keep the first seat, you can do so and exit the councelling.