Metamodels evolve over time, which can break the conformance between the models and the metamodel. Model migration strategies aim to co-evolve models and metamodels together, but their application is not fully automatizable and is thus cumbersome and error prone. We introduce the Variable Metamodel (VMM) strategy to address the evolution of the realization layer of a model-based Software Product Line. The VMM strategy applies variability modeling ideas to express the evolution of the metamodel in terms of commonalities and variabilities. When the metamodel evolves, the models continue to conform to the variable metamodel, avoiding the need for migration. We have applied both the traditional migration strategy and the VMM strategy to a retrospective case study that includes 13 years of evolution of our industrial partner, an induction hobs manufacturer. The comparison between the two strategies shows better results for the VMM strategy in terms of model Indirection, Automation, and Trust Leak.