You are replying to a 4-years old thread, which was related to the OLD -800 version, which PMDG made in a very strange way, by sharing the aircfraft.cfg file between the -400 and the -800 version. This IS supported, that's what I said 4 years ago, but not for both planes at the same time, which was just how the -800 variant was made at that time.
So, I guess you were not referring to the old -800 version, but the new one.
In the today's Live Update, we added internal support for the NEW 747-800 so, it's enough you run the FSDT Live Update, then be sure you RESET the airplane configuration in the GSX airplane configuration editor, to be sure any customization you might have made/downloaded will be removed, and GSX will use its internal config.