Dear student,

No, you cannot do that. You have to choose your category from the very start.

Incase if you do not have category certificate while filling there you might get an option like undertaking form, you fill that and confirm that you will bring the certificate during counseling.

But once you selected General, you may not get the chance to avail the reservation so select your category from very start of filling the registration form.

I hope it helps you.

All the very best!!