Application programming interfaces (APIs) are a common and industrially-relevant means for third-party software developers to reuse external functionality. Several techniques have been proposed to help migrate client code between library versions with incompatible APIs, but it is not clear how well these perform in an absolute sense. We present a retroactive study into the presence and nature of API incompatibilities between several versions of a set of Java-based software libraries; for each, we perform a detailed, manual analysis to determine what the correct adaptations are to migrate from the older to the newer version. In addition, we investigate whether any of a set of adaptation recommender techniques is capable of identifying the correct adaptations for library migration. We find that a given API incompatibility can typically be addressed by only one or two recommender techniques, but sometimes none serve. Furthermore, those techniques give correct recommendations, on average, in only about 20% of cases.