In MDE not only models but also metamodels are subject to evolution.More specifically, they need to be adapted to correct errors, support new and/orupdate language features. The direct consequence of such evolutionary steps comprisesthe problem of managing the co-evolution of existing model instances, whichmay no longer conform to the new metamodel version. This model migration isintrinsically complex and results in a time-consuming and error-prone process if noadequate support is provided. For tackling this problem, we introduce a new techniqueto guide the user in solving migration issues in a step-wise manner. The aimsare manifold, notably the simplification of the migration specification, the reductionof the effort for the evolver, the control of user intervention, and the optimizationof the migration execution itself by allowing in-place adaptation of the existing instances.