1) Since if it is an inadvertent error on your part whilst filling online application, you can always rely upon OBC non creamy layer certificate issued to you.
2) If you online application portal is still available to you to modify your application, you can modify your application by using the rectification/modification option and payment of Rs100-/- as fine for the same.
3) If the online application portal is closed, then you may send a modification/amendment request through registered post with acknowledgment due to the concerned Railway Recruitment board prior to declaration of results and ask them to amend their records.
4) If the exams are already conducted, then also as per HRD ministry guidelines, the Railway ministry will proceed to pursue your candidature as per documents submitted by you at the time of submitting your online application.
5) So the option to change to OBC NCL is available to you till such time the hall tickets for exams are issued and there is no option to change the category thereafter.
Hope this information is useful