Hi, you can make changes in your college as well as the course in between the allotment procedure. If you have completed your first allotment you can change your priority list on the official website and that list will be counted for your second allotment. If you do not get a college in second allotment then your first college will still be there but if you get a college in second list then you won't get the first college you had received. So, choose it wisely. If you still have doubts feel free to comment. Hope this helps.