In a Model Driven Design process, model refinement methodologies allow one to denote system behaviors at several levels of abstraction. In the frame of a model-based testing process, benefits can be taken from such refinement processes by extracting test cases from the different intermediate models. As a consequence, test cases extracted from abstract models often have to be concretized in order to be executable on the System Under Test. In order to properly define a test concretization process, a notion of conformance relating SUTs and abstract models has to be defined. We define such a relation for models described in a symbolic manner as so-called IOSTSs (Input Output Symbolic Transition Systems) and for a particular kind of refinement, namely action refinement, which consists in replacing communication actions of abstract models with sets of sequences of more concrete communication actions. Our relation is defined as an extension of the ioco-conformance relation which relates SUTs and models whose communication actions are defined at the same level of abstraction. Finally we show from an example how a test purpose resulting from an abstract IOSTS-model can be concretized in a test purpose defined at the abstraction level of the SUT.