If market rate is $1800 and they are paying $1500, they are not paying much below market.
Since they have been in the place for 18 years, there likely is a lot of updating that will need to occur. Unless you are very skilled at this, you can estimate at least 2 months for the repairs and re-rental.
If they stay, your 2019 income is $18000
If they move, your 2019 income will be $15600 in rent (ie 8 months at $1500 & 2 at $18000) plus ALL THE COSTS of repairing & renting the unit.
What if the new tenant destroys the place? In one shot, you are going to be out more than the what is the difference in your current income.
Raising long term tenant rent to market rate is short-sighted. Giving them a 20% discount, is generally the average amount, some landlords will go to 30%. They are 17%