Has anyone ever taken a good set of spokes from an early large demountable wheel, cut them down with new tenons, and fitted them to a late smaller demountable rim? I would assume it's possible given the hubs seem to be the same. Thank you.
Why not ! ,I suggest you only change the outer rim only ,you could shave the inner a fraction so it goes further in there for spreading and tightening the hub ,lengthen the tennon end a wisker ,
I have done it with some new spokes that were too long, on my metal lathe. When you turn the tenon, they are not 1/2 or 5/8 inch. You need some squish when pressing them in. My suggestion would be to get a new spoke from Langs and use it as a master to get correct length and tenon diameter. New spokes from the suppliers would be about 1/2 or less that cost plus shipping.
One of the things I wouldn't skimp on is the wheels. 100 year old wood being repurposed might save you some money until the spokes collapse and send you into a ditch.