i have a guitar that was converted to fretless through Malcolm years back. When he was working on the instrument, i had the chance to see how it was done and to do a good job(which Malcolm did), its really not as easy(higher cost as well). Even though its a guitar, still the defretting and finsihing process on bass should be similar.
anyway, as others had mentioned, its easier to get a fretless then to convert a fretted to fretless.
Bro alan summed has basically summed up the main points for the conversion. Its aint some easy job that anyone can do it and doing it well. So if really wanna do it to the instrument, its better to think properly(once fretless, theres no way to go back fretted, unless changing neck), find a reputable guitar tech and saving the money for it(few hundred dollars at least)
anyway, if wanna check for the quote, do check with the respective guitar tech in private thru mails or pm.