Back in the early 80's Peter Cook converted my RD to fretless by grinding the frets down, to give me the idea for the conversion rather the permanant transformation...
Difficult to work back from ground frets! The tangs are still there in the wood, but you have no upper fret to get purchase on. I've seen, oddly enough, a couple of Rickenbackers with the frets ground down to the tangs. I guess you get a nice silver-looking fret marker as a bonus!