This position paper, investigates a way to improve the reusability of legacy tools in specific contexts (defined by specific metamodels). The approach is based on a dedicated language for co-evolution, called Modif. Its associated process involves two model migrations. The first one (Migration), allows to put data under the scope of a legacy tool. The second one (Reverse Migration), allows to put the legacy tool's output back into the original specific context. The approach is generalized by introducing the notion of dependency graph. It specifies the relations between the legacy tool's input and the legacy tool's output. The dependency graph is then used to address some complexities of the Reverse Migration. The improvement is illustrated by the reuse of a flattener tool defined on a specific metamodel of FSM (finite state machines).