Raising rents on long term tenants

[Garrett Blanchard]

In the process of purchasing a 6 unit complex and the rents have not been raised in years. 4 units are 2/1, one is 2/2 and one is 3/1. The rents are as follows... 

2/1 - $425(been this price for 15 years), 2/1 - $600, 2/1 - $650(just raised due to new tenant), 2/1 - $500, 2/1 - $500 and 3/1 - $700(owners daughter lives in). the 2 bdrms should be 650-725 or possibly more and the 3 bdrm should be at least 850. None of the units have leases and are all month to month. 

Question is... How do I go about raising the rents? Raise them all at once to where they should be and hope you don't get a bunch of 30 day notices? I was thinking about offering tenants the option of signing a lease at a lower amount and guarantee the rent not to be raised for that 1 year lease. If they don't want to do a lease, raise the rents say every 6 months until the rents are where they need to be?