I had a friend who converted a guitar to a fretless one (Don't worry, it wasn't one that someday would be a collector's delight.) Rather than removing the frets and filling in the holes, I suggested that he file down the frets and then smooth them with fine carbide paper until they couldn't be felt anymore.

The funny thing was at first he kept playing out of tune because he put his fingers behind the frets which he could still see, whereas he should have put them on them.

I imagine you can make a fretted violin by tying Nylon monofilament fishing line around the neck to make frets. Then you can move them around for different temperament, or remove them for fretless playing. That is the way the frets were on the viol family (except, of course they used gut instead of nylon.)

Murray