We propose an approach to the incremental development of protocol state machines using operators which preserve behavioral properties. We introduce two specializations of the protocol conformance relation proposed in UML~2.0, inspired from the work on formal methods as the specification refinement and specification matching. We illustrate our purpose by some development steps of the card service interface of an electronic purse: for each step, we introduce the idea of the development, we propose an operator and we give the new specification state obtained by the application of this operator and the property of this state relatively to the previous one in terms of conformance relation.