Multi-level agent-based modeling (ML-ABM) requires representing agents at different levels of representation in the same model w.r.t. to time, space and behavior. This paper describes a generic and operational proposal for ML-ABM. First, a generic meta-model for ML-ABM and an associated “morphogenesis” operation are introduced. The generic meta-model allows a modeler to describe multiple levels of representation in the same model, while the “morphogenesis” operation supports agent change of representation level dynamically during the course of the simulation. Second, in order to demonstrate how to operationalize the proposal, we present an implementation of the generic meta-model and the “morphogenesis” operation in the GAMA ABM platform. Finally, we illustrate how our proposal, implemented in the GAMA platform, allows modeler in practice to develop a multi-level agent-based model. To do so, we rely on the famous “Boids” model of Craig Reynolds and show how the modeler can easily introduce a new level of representation of an entity to transform the model into a two-levels agent-based model without having to modify the existing model.