1. A computer-implemented method for modeling a target system, the method comprising: identifying a first block that represents multiple component models in a block diagram model of a target system; displaying a user interface in response to a first user action, the first user action indicating a selection of the first block, the user interface including a mechanism that provides the user with the multiple component models; receiving a user selection that selects a first component model from the multiple component models; incorporating the first component model into the model of the target system using the first block; saving the model of the target system that includes the first component model in a memory; and switching the first block to represent a second component model in response to a second user action indicating a selection of second component model in the user interface, without replacing the first block with a second block representing the second component model.