Hello aspirant 

Thanks fir asking this question .

No , you don't have to register once again.But in case you don't get any college all your choice that you have filled will be considered null and void and you will have to fill new choices.in  case you get your choice you may or may not apply for upgradation.

For further clarification you can refer to the link below

https://www.google.com/amp/s/medicine.careers360.com/articles/neet-counselling/amp 

Best wishes

Hello Priyal Baser, if you had registered for the First round of counselling  for NEET 2019, then there is no need to register again for the second round of counselling of NEET 2019. As if you fails to get a seat in first round of counseling, then automatically you are eligible for the second round of counseling.