It depends on the preference list of your college and course. If you get your number one choice in JoSAA Counseling in any round, then you can not change that college because it was your first preference. However, during the subsequent rounds of counseling, your college and course will keep on getting upgraded according to your preference list and cutoff marks. Once you get the college and course which you want, lock the seat, otherwise, you will lose it in the next round.