How does GunBuster become Dix-Neuf?
I think the short answer is that this is a red herring. Dix-Neuf isn't the original Gunbuster, and the reveal is after the time-skip when the original finally gets back to Earth.

An un-sourced explanation from an /a/ thread says this:


  All the old Buster Machines including Dix-Neuf were built with degeneracy generator, but the former humanity removed it to bury the technology as they branded it to be too powerful. Future generation replaced them with Topless cockpits (Topless being the power-source instead of the generator) and new production of Buster Machines aren't made for the said generators, so only the old generation ones can use them.