Played a couple round in Japan and indeed, you need to select the correct green (eg Bent or Korai) at the time you select the course. If wrong green is selected, than the only way I found to correct the mistake was to exit the round and select the correct course/green.
Press the action button (the main one on the top), then scroll the screen to the last option "X End Round". Tap on "X End Round" and choose one of the proposed option: "Save"/"Edit Score"/"Discard"/"Pause Round". In this case, it will be "Discard"" (no information about the round is recorded).
The round now ends and you can reselect a new course/green.
Ah...I did not had to switch Greens during the round so can't help here. My experience on dual Green is quite limited, hopefully other user with dual green experience will help.
PS: I have a Garmin S2J and I remember that you could switch R/L green while on the course.
