If you flash a new image, everything on flash memory will be gone.
You can backup your chanenl lists/favourites and restore between receivers and images. Use programs like E-chanenlizer, Dreamset, DBedit etc.
If you make a backup image, it is a snapsjhot of the image. When you flash it, it will be an exact copy of when you made it. You can flash the image whenever you want.
If you are manually saving your satellite posyitions, those values are stored in the settings file. It is located in /etc/enigma2
You can restore those between images, but you must stop enigma from running first. i.e issue an "init 4" command.