The duplex I live in was just sold to a new owner. What are the rules for letting the tenants know that the contract will be changed?
As is so often the case with legal matters, your rights depend on the fine print.
If you have a lease -- and you are right to call it a contract -- the new owner cannot change its conditions until its term is up.
If you rent month to month, the new owner is obliged to follow the same notice requirements that applied to the old owner: 30 days' notice (in most states) to raise the rent, change any term of the lease, or terminate the tenancy.