To use :-
java -Xmx256m -cp target/fixretro-<VERSION>.jar org.fixprotocol.contrib.retro.FixRetro \ FixRepository-2010.xml FixRepository-2010_Retro.xml
The resulting FixRepository-2010_Retro.xml can be used with FIX Converter to allow the conversion between FIX.4.2 and this XML representation of FIX.4.2.
The resulting FixRepository-2010_Retro.xml can also be used to generate XML Schemas. Copy it over the original FixRepository.xml alongside the various FIX_*_phrases.xml files. Use Lasalletech FIX Explorer 2.2.2 (or later) to File -> Open FixRepository.xml, and then File -> Export... FIXML Schema. You can then validate these XML messages against the generated fixml-main-4-2.xsd file, and the files referenced by it.
Although Lasalletech FIX Explorer is mentioned in this document, other Unified FIX Repository editing tools and FIXML schema generation tools should work too. Its just that this is what I had handy at the time to test with.