For most cases it should just be a drop in replacement. IIRC they promise not to break the API between point releases (except tf.contrib.* which may change or disappear entirely...)
0.9 wasn't production ready and they didn't guarantee backward compatibility until 1.0. So nothing to change from 1.4 to 1.5, maybe you will have some warnings about features that will change in the future, but it will work.
