On old arduino's there were jumpers to select the power supply but I don't think they are present nowadays so I'm not sure how it would work , but if you were doing it on one of the old boards , you could incorporate a slider switch to toggle between the jumpers or you could use , I guess, the arduino itself to switch as long ad the space between the off and on is short enough but it might take too long in arduino time and you may well find it wont work that way. in that case , you could use another arduino to do it for that one but that is long winded and expensive...
As for the auto reset function , that is settable in the arduino software. Scout around the options menus.