Hi aspirant

you can change it 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 documents like OBC/SC/ST/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 .Yet nothing is specified .You can read more about everything here that is yet mentioned by JEE Advance committe:(Go to page no 13)

https://drive.google.com/file/d/1xTk8FAJ7IYe7kkHgyTr-vaoatefQIzeY/view

Check the form fill up procedure here:https://engineering.careers360.com/articles/jee-advanced-application-form

Check the eligibility criteria here:https://engineering.careers360.com/articles/jee-advanced-eligibility-criteria

You must be well aware of how to fill the form and what will be required.

This year more than 2lakhs candidate will be qualified due to the adiition of EWS category.

Hope this was helpful.