First you open the source file, then you call `Clone` and pass the new transfer syntax as parameter.
var oldDcm = DicomFile.Open("oldFile.dcm");
var newDcm = oldDcm.Clone(DicomTransferSyntax.....); // whatever you want
newDcm.Save("newFile.dcm");
you might add the usings manually, if your IDE does not recognize. the `.Clone()` method is an extension method and lives in namespace `Dicom.Imaging.Codec`