Hello dear

You will get the college that is possible for you at your rank. It won't be that you may get any other college. So even if you put an option that you can't get (based on previous years cut offs) above the one that you are getting, it won't slide up and your choice will remain the same. As in, in your case, you are getting IIT Gandhinagar, so you will get this as IIT kharagpur is not a possible option. Also, if according to last year's trends, if you can get Varanasi then it will slide up( you need to choose the float option for that, if you select freeze, then your seat gets fixed). 

I hope this helps you. All the best!!