make a switch node. (named it Switch_input in this example)
clone it and insert wherever you want to switch in upstream.
make a several write node

in "before render" in python tab on write nodes, write one of code below to each write node,

nuke.toNode('Switch_input').knob('which').setValue(0)
nuke.toNode('Switch_input').knob('which').setValue(1)
nuke.toNode('Switch_input').knob('which').setValue(2)
....