There is a VBA example in the 10.0.3 developer help at this link. I don't know why the latest version of the help doesn't include the IMMObjectClassConversionTool, but it looks like the ProgId for the conversion tool is still in the registry with the 10.2.1b release. More than likely, the VBA code could be rewritten in C# and it would work with current ArcFM versions.
The following snippet should help point you in the right direction.
public void ConvertClasses(IFeatureDataset featureDataset)
var conversion =
var container = (IFeatureClassContainer) featureDataset;
foreach (var featureClass in container.AsEnumerable())
conversion.ConvertClass(featureClass, mmObjectClassConversionOption.mmConvertToArcFM, false);