landlord newbie under contract for leased duplex

[Kendal Glover]

Hi everyone! We are in our option period to buy a duplex in fort worth, TX. Both sides are leased at low rent with no security deposit. We are buying the property well below market value. The leases look fairly thorough. My question is, how should we go about getting the tenants to sign a modified copy of the lease stating us as the new owners? I know we have no right to change the terms of the lease (esp rent) until it expires. And do we need to do this before our option period is over just in case they won't sign? All the signs point to them being fairly good tenants but I don't want to get into a situation where we can't even enforce the terms of the lease because our names aren't on it. Thanks so much in advance for anyone's help on this!!