If you have valid documents to prove that you belong to EWS category, you can change your Category to EWS for JEE Mains April application. Because EWS reservation is for Candidates who belong to General Category and whose annual income from all sources is less than 8 lakhs. So if you fulfil the criteria, you can select EWS Category during April session. There wont be any issue because EWS is Category taken out of General Category. So even if you have opted General Category as your option in January session, you can still select EWS as your Category for April session.