This report present the formal basis of a migration round-trip aiming at promoting the reusability of legacy tools in specific contexts, defined by specific metamodels. This round-trip is based on Modif, a dedicated language for metamodel evolution and model coevolution. The first step of the round-trip called initial migration allows to put data under the scope of a legacy tool. The second step consists in applying the tool itself. The last step called reverse migration allows to put the legacy tool's output back into the original specific context. This migration is made possible thanks to the notion of dependency graph. It specifies the relations between the legacy tool's input and the legacy tool's output.