How to raise rent when unit is well below market value

[Jourdain Francine]

My fiancee and I are buying a duplex in Brunswick, Maine that has a great location.  Walking distance to multiple local restaurants, Bowdoin College, public library, park, grocery store, etc. My fiancee and I will be living in the downstairs unit. The tenants that live in the upstairs unit are supposedly very nice, do not complain and keep the unit in great shape. The upstairs unit is a 5 bedroom, 1 bath that is currently renting for $1,150 with heat included (old house so separating heat will be a project) which is far under any comps in the area (between$1,500 and $1,900). My hopes would be to rent it for $1500 with heat included, but I can't see the tenants staying if I increase the rent by $350 a month.

We are looking for suggestions on how to handle this. Do we take the cut in rent because they are good tenants or just risk them leaving and hope it gets filled at $1500?

Thank you in advance!