Amend taxes to update NAICS Code?

I'm currently in the process of applying for a grant that requires a certain NAICS code. I'm a freelancer with multiple jobs and streams of income. I operate as a small business for my art and handmade goods, but am also a graphic designer. In previous years, I roped my art income with my design income, since I didn't have a formal income form for my art business, and I technically do design everything and it was just easier. My advisor who's helping me on this grant suggested I file amended returns to update my income for my art business and therefore meet the requirements as an arts NAICS code to claim reimbursements through this grant.

 

My taxes have been a major pain point over the years I've been a freelancer and I'm afraid of making them even more confusing by amending my returns, especially without formal income forms. Any advice would be highly appreciated. Thank you.