1. Hello Mayank,
2.  
3. You don't have to submit your old CBSE marksheet, you will only need to provide your old CBSE  Rollno,  your 5 digit School Number, and  your Examination center number given in your Admit card

THANK YOU 

Dear after your 12th class result, after some time approx 10-15 days CBSE issue a notice in which you can fill the option of improvement exam and fill nominal fee for it according to the number of subjects you want to improve. After a result, you get updated marksheet of CBSE board.

Best of luck