Extensive literature exists on how software models are created, maintained and validated, but little attention has been paid to what happens to models during their practical use. Often, a model is created by someone and then used by someone else, so that model extension may be necessary to adjust the model to particular usage needs and context. Model extension usually involves making changes to the base model, but these changes are likely to produce a resulting model that is no longer Liskov-compatible with the base one. To avoid this, many extension approaches restrict what changes are possible once the model has been finished, limiting the expressive power and reach of potential extensions. In this paper we present a new approach to model extension, based on the ConML modelling language, which puts very few limitations on how a model may be changed. A set of reinterpretation rules are used so that a human or tool can easily recast an instance model conforming to the extended model as conforming to the base one. By using this approach, expressiveness and reach of model extension is higher, while preserving Liskov compatibility with the base model and facilitating model reuse and application.