How does GunBuster become Dix-Neuf? At the risk of going into anime physics, how does GunBuster become Dix-Neuf?

My understanding of the series is that GunBuster, the first Buster Machine ever created, was used in the final battle of GunBuster to drop off its Degeneracy Generator. It then limped back to Earth and appeared 15,000 years later

Then in DieBuster timeline you eventually figure out that Dix-Neuf, a Buster Machine with a number in the teens, has been slowly upgraded and rebuilt over a long period of time and is missing its own Degeneracy Generator. Because of that I assumed that Dix-Neuf and GunBuster are the same.

However, how did GunBuster become Dix-Neuf if GunBuster is still floating in space? What's the connection between the two machines?
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.