Rental Property question

[Shawn Dandridge]

I just bought a duplex in Memphis. 38109 area code. Currently rents for $200 per side. $400/mo total. After all my expenses. I'm left with about $150 a month in profit. The rent is insanely low. Section 8 is paying closer to $600 for a 1 bedroom efficiency in the area. Tenants have been in the units about 3 years. I will eventually raise the rent. I'm looking to raise the rent to about $375 per side. Should I do it all at once or a gradual increase over a year or two. Any thoughts would be appreciated. I should note that cap rate at current rents is 32%.