To avoid the lock-in problem in service-oriented software, existing interface-decoupling mechanisms focus on identifying high-level service mappings, which are not necessarily applicable on translating actual data. Based on the fundamental data-translation process, its successful outcome is guaranteed if mappings are low-level, i.e. they satisfy schema constraints. The problem is that if similar services have been identified based on high-level mappings, then schema constraints may be violated. To overcome this problem, we propose a proactive approach to identify similar services that satisfy schema constraints. In particular, our approach follows a composite workflow (instead of existing hybrid workflows), which determines schema constraints (service documents do not specify them) and estimates service-translation cost (actual cost is not available) as a function of ensured schema-constraints. We compare our approach against a state-of-the-art service-similarity approach on benchmark services and the results show that high service-similarity does not necessarily imply low service-translation cost, the bidirectional nature of service similarity can be misleading, ensured schema-constraints improve service similarity, and estimated translation-cost is very close to actual cost.