Model-driven engineering (MDE) provides gainful solutions to software development, maintenance and evolution. Model based standards and techniques emerged and proved useful to develop new applications but also for reengineering legacy systems. Unfortunately MDE is subject to frequent new standards and upgraded releases which lead stakeholders to proceed costly maintenance operations of the software modules they depend on. The existing techniques suffer from limitations and require new solutions to ensure a better adaptability and flexibility of the tooling. We propose a solution to modify metamodels already in use without rebuilding completely the software product with graphical facilities. This solution includes an improved technique of virtual extension of meta-models with Facets and end-users facilities to handle the model mappings. An open-source implementation, including user graphical interactions, is available on-line.