It is a common requirement to move between RDF Schema models and OWL ontologies. For example, projects may be based on legacy data in RDF Schema. If OWL is the target language and you want to use OWL's expressivity, then you may want to convert the RDF Schemas to OWL. Later, if you plan to export your project to RDF Schema users, you can convert it back to RDFS. Note that in this direction you may lose information such as restrictions.
To do either direction in TopBraid Composer, select Model > Convert between RDF Schema and OWL... from the top-level menu. This will take you to the following wizard.
On the first page you select the target language. If you currently have an OWL file, then your target will likely be RDF Schema.
On the second page you can select which classes shall be taken into consideration for the conversion. Depending on the selected target language, owl:Classes will be converted into rdfs:Classes and vice versa.
On the final page you can select which properties to convert. Again, depending on the target language owl:ObjectProperties and owl:DatatypeProperties will be converted into plain rdf:Properties and vice versa.
After the wizard has completed, your in-memory model will have changed. Language elements that are not present in the target language, such as restrictions and union classes have been deleted from the model. You can review the changes and either save the file under a new name, or undo.