I think you answered your own question…
Keep in mind that we are just about to receive our first FarmBot - take my input with those grains of salt…
Having said that…
I think you would need:
- another set of tracks (including the mounts)
- a second beam for the gantry
- joining plates (these are a fairly good portion of the cost)
- a “redo” of all the wires, cables & hose, including added length to the cable carriers and additional cable carrier mounts.
So, in my opinion, it would not be a “simple upgrade”, but would be less expensive than a new kit.
Another “bonus” would be that you end up with some spare parts and stuff that you could do some R&D work with. As you said, I think all the electronics, motors and z-axis stuff would be re-useable.
All in all, it might be feasible, as long as you go into it knowing the above and are ok with that. It might be a good idea for the FarmBot folks to put together and market/sell a “FarmBot Genesis XL upgrade kit” - they could put all the pieces together, include instructions and possibly pass on some cost savings.
Best of luck in you future “growing” endeavor!