While this thread title is about the switch, for switching power source from fridge from 12v to 24vDC, I have no personal interest in the switch itself, and I have already typed what my personal solution to this function would be if it were an option in my own system, or I was setting it up for someone else..
My concerns with doing such a thing, would be the possible destroying the Danfoss compressor controller which costs some 350$ to replace, which is likely more than half the price of the fridge itself.
The safest way to ensure that there is no risk, at all, is to make sure that when switching from 12 to 24vdc or visa versa, is that the fridge thermostat is turned off and compressor shuts down. Then after possibly a period of time whose duration is unknown, move the power feed to the compressor controller to from 12 to 24 or 24 to 12v, then turning the fridge thermostat back on.
I have absolutely no interest in a switch, or anything automatic to accomplish this task, as I see nothing but a possible blue smoke release from multiple areas on setting up something which can be accomplished so easily, and safely, manually in 30 seconds? instead of 2.
Now as far as if a quick switching from 12 to 24vdc or back, and how the compressor controller would handle that, I have no idea. It is not a risk I would be willing to assume does not exist, nor that this 'convenience' is worthy of effort or expense. I fully understand other's opinions on the deisre for this convenience, are different.
I do have an interest, though well over my head, regarding the electrical components inside the compressor controller, and how they they would handle, or not handle, and abrupt change in DC input voltage. I will ask my knowledgable source on this possible risk, but that will only take a few moments of his time to type a response. If he chooses to do so. He has chosen not to, or forgotten to respond to some of my questions before, I am not going to ask that he dissect parts and take time from his life and his personal projects where he desires to spend his limited free time.