No problem, if you tick the " clear data" option, you can downgrade,/upgrade any rom you want, but there is 2 notices for you:
1. If your phone was shiped with rom version G ( international rom ), you can flash rom G or A on your phone
2. If your phone was shiped with rom version A ( rom for chinese ), you can only flash rom A on your phone.
- " Shiped " here that's mean your phone must be bought from Meizu original distribuiter, not from other user.
- How to know your phone is running on G or A version? easy, just go to " Setting -> about phone "
- You are still able to flash rom G on the phone was shiped with rom A, but there are more difficult steps must be done before flash rom and it base on your phone model. If you want, ask google .