Python(Nuke)

[python {nuke.thisNode().input(0).knob('file').value().split("/")[-1].replace("prefix","")}]

changeFrom = "a"

changeTo = "b"

sN = nuke.selectedNode()

for i in sN:

    if i .Class() == "Read":

        path = i.knob('file').value()

        re_path = path.replace(changeFrom,changeTo)

        i.knob('file').setValue(re_path)