If you were changing from oil to propane, there would not be a problem. The determining factor here is the chimney. Chimneys for propane systems are relatively small, while chimneys for oil systems are much larger. The oil system also requires more space. It is possible that when the house was built the chimney was designed for an oil furnace. If that is the case, there is no problem converting because the chimney will be large enough. If the house you are thinking of buying was built with a chimney for propane, there could be problems changing to oil. There is a possibility that it can be done if you purchase a new oil heating system that is very efficient. If your house does have a smaller propane chimney, have a heating specialist take a look.