Raising rents on inherited tenants

[Matthew Insley]

I just bought a 4 plex that is old but in pretty good condition for a C+ neighborhood.  I know that the rents are lower than what I could probably get, but I have decent tenants in place since before I purchased the property.  The first issue is that even though all 4 units are very comparable, they are not renting for equal amounts.  I am receiving $450 for 2 units, $410 and $400 for the other 2 units.  2 leases are expiring in April 1=$450 per month and has already expressed no desire to leave, 1=$400 per month with an unknown intent.  I have one tenant on a month to month agreement for $410 per month and has expressed concern of new ownership and that she is comfortable with where she is (she has lived here for 12 years).  I have assured her that everything will remain the same for now, and she will be given 30 days notice for any changes.  The final tenant is renting for $450 and his mother pays his rent every month and he seems like a loser, but other than smoking in the unit, he seems to be an okay tenant.

I obviously am not going to do anything until the spring so that I don't lose any tenants at a time that is very difficult to get new tenants.  I would like some advice to how and raise rents on the current tenants with somewhat of a low likelihood of losing them.  For this property, I would be happy with rents close to $500 per month.  Please help! This is my first rental and could use some expert advice.  It is also my first bigger pockets post. Thanks!