what happens when multi family tenants have no leases?

[Isaac El]

Looking to purchase a multi family property (small 4 units, not rent controlled, not rent stabilized) in New York (don't know if state matters, but I am sure it does because different states have different rent/eviction laws) The current tenants are all paying the rent, however the current landlord has not raised rents in 20 years! So these tenants are paying super low rents, the building is barely breaking even, and needs some costly repairs.  Discussing with landlord to purchase property, question is, how does it work if I buy the property and these tenants have no leases? Am I allowed to come to them and say "we are making new rent?" or "we need to increase your rent" for example one tenant is paying $620 on a unit that is market rate at $1500, appreciate all advice.