But Cuix files are actually Zip-files that contain CUI files and bitmaps. The same system that Docx and Xlsx uses. So that is quite common and I can't find a reason to change this. Only Autodesk might come with new tags or options inside the CUI system.
Yep and the CUI files inside that CUIX is in fact XML files (again similar to DOCX/XLSX/ODT/ODS/etc.)
And all that an XML file is is something similar to an html file, with the exception that its tags define a structure for data instead of how to display a web page. So as long as ADesk don't rename one of those tags inside the CUI backwards compatibility might be easy to keep. If they add a new feature into the CUI (e.g. the ribbon panels are new from 2009 onwards) these "should" simply be ignored when opened in an older acad. At least, that's how it "should" work if ADesk follows XML rules and general common sense.
I have found though that the CUI easily becomes corrupt if you try and edit it manually (even using an XML editor to keep its XML rules correct). I've even just edited a comment in the CUI and still this caused an error when loading it. So I'm not too sure how ADesk has implemented CUI reading. It doesn't appear to be a standard XML stream reader, as most other programs use. If it was then stuff like comments would not cause the entire XML to fail. So I can't say for sure if some future version of CUI might cause total incompatibility with an older acad.
