New owner looking to change a pre-lease before it starts

[Brandon Flood]

I am looking to purchase a multi-family house that currently has tenants living in one of the units. The current owner is charging them well below market rent, and I was told that they signed a year "pre-lease" through June of 2019 (so doesn't start until June 2018) with an minimal but insufficient price increase. I am wondering what the laws are around a new owner changing the terms of a lease that hasn't yet gone into effect.

I've already considered the prospect of lowering the purchase price by the amount we are missing out on, but haven't proposed that idea so I'm trying to cover my bases. Thanks in advance!