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? 

[Todd Powell]

@Garrett Blanchard
We bought a 5 plex two years ago. The seller told us that he owned it for 15 years and rents were low and could go up. Yes, he owned it 15 years and, Yes, his rents were lower. But, he failed to tell me that just a month before he had already raised rents. In the end, his rents were still below market. They averaged $625 per unit and we raised them to $850 and also gave the tenants the garbage bills. The tenants kicked and screamed, but what I had to tell them was it was a business and that we paid a lot for the units and have to make it work. For fun, I asked one tenant, "how much do you think we paid for this building?" He answered, "I hope you didn't pay over $150,000." I chuckled and said, "that's why I have to raise the rents, as we paid $375,000." I came in with rent increases and added value for them with new granite in every kitchen, new windows, gutters, etc.
Its a business and due to drama after two years we sold it last month for $545,000 on CL with zero realtors or escrow fees! Moral of the story, do not be afraid of running your business, OR tenants will run you out of business !