If the wiring is in good shape and there are no bare spots (which as far as I'm concerned negates the "in good shape") and the connectors and connections are properly terminated I really wouldn't go to the effort. Technically, a 2 prong plug is still "legal" or I wouldn't have so many modern appliances with them including laptops.
My cousin is an electrician AND she's a quilter and vintage machine collector and has also never mentioned needing, being so inclined or concerned that they needed to be brought up to modern standards. Only that the wiring be properly done with no exposed wire.