Well for CFL there is just the ballast since they will specify electronic gear, so it would be simple to remove the old gear tray and fit a new one with a CFL ballast. The other change would be a new "bung" for the optic with a CFL rather than ES lampholder.
If I was in charge...
, to ensure there wasn't a mix match of lanterns, I would fit new CFL lanterns replacing SOX which would go for scrap, the ZX1s would be removed and taken back to the depot for conversion and then reuse elsewhere. It would only take a few minutes per lantern to convert (using prewired Urbis "kits"), and it would be quicker to do at the depot with them going in the stock pile to be used again. That way would make lantern conversion quick and easy "on the street" and would ensure that entire streets gets converted in one go and that the same type of lantern (and optics) are used per street.