If Mr. Johnston has a warranty on his work and you are still under warranty, I would call on him to do it.
It is really not that hard though if you want to tackle the job! If you are skiddish, take photos every step of the way! Besides - we are here to help you through any bind.
First - UNPLUG THE MACHINE!
It is possible to do it with the door hanging, but I would get some help.
If you can disconnect the live door wire from the harness, then yes it is easier removed. Have a thick moving blanket ready to lay the door down on!
Either way - remove all of the door liner screws (the liner screws should all be the same). If leaving the door hanging, leave the top screws until last and have hands securely holding the liner for the last few screws up top - it is heavier than you think, so have those hands holding tight.
Remove all of the insulation and place it on the ground in the same configuration as you remove it (I place the insulation on top of the liner as it is removed and in the proper location in relation to the liner). You should not have to remove all of the insulation
Disconnect the wire nuts (or cut the wire if there is enough slack to recreate the connection). Pay colse attention to which wires are connected together (or take a clear picture)
Replace the ballast and reinstall everything in reverse order making sure to get each screw through the liner and corresponding gasket hole.
NOTE: I like to install (or re-install) all of the screws like you would a wheel on a car (in opposites).
"light bulb" - Has anybody ever mounted the ballast under the bottom shelf on the door beneath the liner? It certainly wouldn't get in the way - would the heat from the compressor affect it? Would most definately make it easier to change - will call it an 81E