20 year tenant or unknown for more money

[Dawn Burwell]

My first buy and hold.( 2 houses for 49k )   Im inheriting a tenant that has been in place for 20 years. Her rent has not been raised since 1984. She currently pays 450 a month. ( plenty to cover my mortgage -440 - on both houses on the property)  However.  The rental amount for the area dictates that her rent should be at 700.   Seems a good idea to keep a long term tenant that has taken very good care of the house. BUT... I could really use the extra income on this.  As I have 2 houses in this deal to take care of . 

I dont HAVE to have it. Im house hacking my way off of disability ( with a roomate too) , and just need a way to have a home for now. Im torn between keeping a reliable tenant.... or getting a new one that would pay market.  Im not even sure I am allowed to raise her rent THAT much in a shot or even a year.   ( Pennsylvania )  

How would I even approach this poor old woman? Maybe I should just be grateful to inherent a good tenant??