If you plans to register for JEE Main April exam, then only you can change the category for JEE Main from General to GEN-EWS.  When the application forms for April attempt are out, fill it, change your category and pay the application fee. If you forgot to change the category while filling it then wait for the correction window to open. When the correction period starts, then change your category from General to GEN-EWS.

If you do not fill the application form for JEE Main April attempt, then you will not be able to change the category (in JEE Main Jan Result).