Well,there's always a risk,mail CAN get lost.but what's the alternative? I don't think,just a picture of the new gc would be enough. Maybe depends on where you are / what country at that time? But,you'll also have the approval email,you might be able to go to the embassy abroad to get a stamp in your passport? Or,if applicable, apply for a re-entry permit before traveling? Try to find out,if that's an option,the re-entry permit!