import nuke
import nukescripts

def swapInOut():
    n = nuke.allNodes()
    for i in n:
        if i.Class() == 'Merge2':
            if i['operation'].value() == 'in':
                i['operation'].setValue('mask')
                nukescripts.swapAB(i)
            elif i['operation'].value() == 'out':
                i['operation'].setValue('stencil')
                nukescripts.swapAB(i)

swapInOut()