@Tod ********
sharing my personal experience .
You need to cancel Non B extension first from counter M. Requirements for cancallation are
Work permit, passport, DBD updated from employer with closing accounts, last 3 months personal income tax slips, SSC paid proof, letter from employer to switch cancel extension.
Before going to counter M better consult L and clearly discuss what exactly you want. Officers were totally confused whether to cancel Non B first or not. There was a hell confusion between the departments due to which i was overstayed..
Being employer myself i clearly mentioned in application letter that i only want to switch from NON B to Non O and carry on with work permit . They force me to cancel non b which i think was totally non sense