Hello there,

According to my knowledge ,you can select the maths standard  only one time . After selection you cannot change to standard type,  The CBSE had for the first time introduced two-level Maths (Standard and Basic) in Class 10 . Basic Maths is for those who do not want to pursue the subject further and the Standard Maths is for those who want to study Mathematics as a subject in Classes 11 and 12.

So I think you had not did any mistake by choosing Standard Maths ,This will going to help you in future while Getting Admission .

Hope this Information will help

Good Luck!