How should I raise rents on an old tenant?

[Adam Hoak]

I have a duplex that I would like to buy. It currently has a vacant side and a rented side. The rented side is for $575 and below market average which is $750. She is elderly (mid 70s) and has lived there for 13 years with no rent raises. If her rent was $675 I could make the numbers work much better for a house hack. It would cover the mortgage and insurance 100% but nothing more. How should I raise her rent? Also, it feels immoral to raise rents with the COVID-19 situation at hand. Any thoughts, please?