From a model, we generate multiple running instances. When the model is updated, so are the instances. This allows immediate feedback from user-interaction, while still developing your model. The model is a state-description and has to be well-defined, because of this we manage it with the type-safety of Gleam. The instances each contain an independent state. We want to migrate this state when the state description changes. Because of this, we generate the code for the instances in Core Erlang, and use BEAM facilities to upgrade the instances while running. Naturally, for the largest part, this presentation will be a live demo.