Greetings !!

You may fill CHSL Application form according to the current Valid OBC certificate in your hands. There won't be any issues as both castes are same. At the time of document verification, if any clarification needed you may produce one authorization letter from the concerned authority to prove both the Castes are same. Start preparation early, stay focused, dedicate at least 4 hours for daily preparation, You will be able to clear the exam. If any doubts, pls feel free to ask !

Wish You Good Luck !

Thank You !