I have Global version, M792H, so I'm not really able to properly help you, but on this forum there is similar thread with yours, http://forum.flymeos.com/thread-45321-1-1.html, where the user complains about not being able to revert to Chinese ROM (A) from Global. His thread is unanswered, but maybe it helps you finding that could be problems changing A to G and back to A ROM.