When you get a seat in JOSAA counselling, you can either accept that seat and withdraw from counselling by freezing your seat. You can also accept the seat and continue with the counselling by using the float/slide option. This way if you get a seat which is above in your preference list, then you get that seat and the previous seat is lost.

Either way, once yoyu get a seat, no one can take that away unless you do not accept it by reporting at the reporting center.