Hello ,

No you can't change cateogry in JEE Advance direcly but by declaring that you don't belong to OBC-NCL you will be considered as General category candidate and there is another way while filling for JEE Advance you can give declaration in lieu whose format will be available at JEE Advance website in which you have to declare you don't have OBC-NCL certificate now but you will upload it before deadline that is given on JEE Advance website.

So, you can give declaration and try to get OBC-NCL certificate  with stamp and signature of SDO from your district before the deadline given at JEE Advance website.

Also you can change cateogry to general by declaring that you don't belong to OBC-NCL but not directly.

Hope this will help you.

Here are some FAQ for JEE Advance 2020 to clear your doubts :-

http://www.jeeadv.ac.in/faqs.php