Incremental development is now state of the practice. Indeed, it is promoted from the rational unified process to agile development methods. Few methods however guide software developers and architects in doing so. For instance, no tool is proposed to verify the non-regression of functionalities, modeled as behavior specifications, between increments. This work helps to incrementally specify software functionalities using UML state machines. It provides an on-the-fly evaluation of a specified behavior as compared to that of previous increments. The proposed contribution is based on two formally specified relations that are proved to preserve refinement when composed. Architects and developers are free to choose their preferred behavior specification strategy by iteratively applying them, so as to develop the required functionalities, having at each step the benefit of a formal non-regression checking to guide the global specification process. Our proposal is implemented in a proof-of-concept tool and illustrated by a didactic casestudy.