Hello Candidate,

If you're talking about that the candidate has already given his JEE Mains examinations and the results for that particular session have been out, then if you change your category from general to generals ews then it would not affect the result or rank of that previous attempt which you have already given and whose results have been declared.

By changing the category to general EWS you can attempt the examination in the the next sessions, where you will be treated as a a reserved category student rather than a General candidate.

Hope you found it helpful. If you have any further queries feel free to post it here!!