Santosh, the correction window is the last chance for you to make any changes to your application form. Once you select a Category and the correction window is closed, NTA will not entertain any requests for changes nor does it make any changes on its own to the application form. So, answering your 1st question, in case your EWS Certificate doesn't come through you cannot change the Category and cannot make any additional fees. Your Category will remain EWS alone. Now regarding the 2nd question, in case the EWS certificate doesnt come through, your Category will remain and be considered EWS and you will not be considered General Category. The merit list is prepared on the basis of your marks you score in NEET and the Category you have chosen during filling the application form. There will not be any modifications in the merit list based on the later changes of your Category.