Is there any obligation upon the property owner if he changes the broker after listing a property with him that has not been sold, and is the first broker entitled to anything?

If the broker does not fulfill what was agreed upon, there is no impediment to terminating the contract with him, because brokerage is a form of ju'alah (a contract for a reward), which is a permissible contract that can be terminated by either party. The broker is not entitled to anything as long as he has not started the work. However, if he has started the work but not completed it, he is entitled to a fair wage (ajr al-mithl) if the termination was initiated by the one offering the ju'alah.