This is a fairly common procedure. Taking a fretted bass and converting it to a lined fretless. The fret slots are filled with a piece of maple which makes a great marker.
The following information is not meant to be instructional in its content. This work should only be done by a competent, professional luthier!
The patient is a Tobias 5 string. The fingerboard is Pau Ferro.
The frets are removed by heating them up with a soldering iron and a small bit of solder for better heat transference. This will break any glue bonds that may be holding the frets in place.
Great care must be taken not to remove chunks of fingerboard from the fret slots.
Please don't try this at 3:00 AM Saturday morning after you've returned home from a gig that you wished you had a fretless. We've seen that.