You can get that seat in the further rounds if your rank comes under the cutoff. But if you have not added NIT Durgapur at the beginning of the first round, then there is no way to get that college because you can't change your choices mid counseling. If NIT Durgapur is below NIT Silchar in the choice filling then you won't get the seat because the system doesn't check anything below your allocated college.

Hello there. If you want to change your seat then you have to choose the option of float by which you can keep the current seat and can look for other college in upcoming rounds.