Hello aspirant.

If you get allotted a particular medical/dental college and you opt for up-gradation as you want to try your chance in another college of a higher priority, you will have to leave the seat of the college you get in. However, if you do so, you won't be able to get a seat in the same college again in the second or even mop up round. Thus, in your case, if you leave a seat in BDS Sundernagar in the first round of counselling, you will not be able to get a seat in the same college.

To know your chances of getting into a particular MBBS/BDS college, you may want to try our college predictor tool https://medicine.careers360.com/neet-college-predictor

Hope it helps!