To save you some time and confusion with mGBA, the standalone emulator
still uses *sav files like VisualBoyAdvance and I have tested it Astro Boy: Omega Factor with the save file I have for that game and I was able to continue on where I last left off rather than start anew, so switching from the VisualBoyAdvance to mGBA as standalone emulators is no problem. However RetroArch
uses the *srm format to store save data for most of the systems it emulates. If worst comes to worse, I guess I can tell my LaunchBox set-up to use the standalone mGBA instead of RetroArch's mGBA core to replace VisualBoyAdvance-M. Also, I tried an unmodified ROM of Boktai on VisualBoyAdvance-M and it gave me some Windows error messages when I tried to set the in-game clock and timezone, then I got a "Solar Sensor is Broken" message the minute the game started and it stopped playing.-EDIT-
Okay so I just found that you can re-name the extension of the *sav files from other emulators and they can work with RetroArch. I tested with an old save that I have of Castlevania: Aria of Sorrow and all of my progress is still there from VisualBoyAdvance-M. I guess I can close this thread now.