Nope! During the Admission/counselling round, the category can't be changed. The main reason behind it is that during the results, the category wise ranks are provided as a result, the declaration of the category is required during the form fill up.

So you didn't apply for admission with EWS if you haven't provided it during the form fill up.

For more information regarding the MHTCET you can consider the link below:

https://www.careers360.com/exams/mht-cet

I hope this helps. Wish you a great day!