Raising rent on a newly acquired unit with current tenants

[Michael Karl]

My wife and I are getting ready to close on a 3 unit rental property in a suburb of Buffalo NY.  All three units are currently occupied, 2 of them are month to month and one is currently on a 1 year lease.  All three are currently being charged under market value for the rent.  I have a couple questions for the landlord community out here on BP.  When is the right time to raise the rent? How much is reasonable to raise it at one time? Should I attempt to get the month to month tenants to sign a 1 year lease? And is it acceptable to charge them more for rent if they want to stay month to month?