For most of the cases there will be no options for changing the category during counselling. Although there can be options depending on the organizers of your counselling. But in most cases/examination there are no such options available for changing of category. To be sure you can contact the helpdesk of the organizers.

The eligibility for the EWS certificate are:

• The annual income of the family of candidate should not exceed 8lakh per annum.
• The family should not hold 5 or more than 5 acres of land.
• The family shouldn't have a residential area more than 1000 sq feet.
• The family shouldn't have a residential plot (in notified municipalities) of an area 100 square yards or more.
• The family shouldn't have residential plot (other than in notified municipalities) of area 200 square yards or more.

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