Tenant lease problem

[Justin Mayne]

Hello everyone. I'm about to close on a property in Oklahoma City, OK. The property has 3 units, one main house, and 2 detached garage apartments. One of the tenants first rented out the main house and then added the upstairs garage apartment a few months later which required a new lease to be made with the seller of the property. Since I'm buying the house with a VA loan, The tenant had to move out of the main house so that I can live there.

When the seller first asked me if I wanted the tenants to stay after the property was sold, I told him that I would like for them to stay and I will create my own lease for them.

The seller made an agreement with the tenant that was very vague and not specific. The agreement stated that her lease in the upstairs apartment would not be affected by the sale of the property. The seller also paid the tenant $5000 to break her lease.  

The lease the tenant had was for both units, the main house, and the garage apartment. In that lease, there were no specifics on rates per unit. The first lease she had with the seller was for the main house at a rate of $975 a month. The second lease they created was for the apartment and the main house at a rate of $1400 dollars a month.

The tenant now thinks that she does not need to sign any new lease. She believes that the lease transfers over to the new owner without needing any documentation. She also believes that her rate will be $425 a month by deducting the original lease from the second lease.

My question is, do leases automatically transfer to the new owner? Does the agreement she had with the previous owner stay intact or even matter once the property is sold? 

Thank you for any input or advice you may have!