You can find out the old registration ID and you can log in and edit your profile while applying for the examination. You will find the option for editing your details there while logged in. Log in to the account and try to apply for an examination and you can edit it nothing to worry about it.

To know more about SSC CGL visit the link below:

https://competition.careers360.com/exams/ssc-cgl

I hope this answer helps. All the very best for your future endeavors!