There might be a chance to rectify your OBC category mistake during WBJEE, but it's difficult to say for certain without referring to the official WBJEE information.

Carefully review the WBJEE information bulletin, particularly the section on application form corrections or discrepancies. This document outlines the official procedures for handling mistakes in applications. You can find it on the WBJEE website.
The WBJEE helpline is likely the best way to get a definitive answer. They can clarify if there is  a window for rectifying category mistakes and the process involved. You can find the helpline number on the WBJEE website.

https://www.careers360.com/exams/wbjee

I hope it helps !