Hello Aspirant. 

You can change your category as you will have to fill the application form again when you are applying for JEE Advance .At that time you can fill the form for EWS section but you must have a certificate for that .At the time of form fill up you will have to upload the EWS certificate .It usually happens that you need to fill the entire form again .

But it might happen you will be not allowed for EWS section as you have not filled it in JEE mains .You should wait for the application process to start .

Hope this will help you. 

All the best.