Looking online, opinion seems to be split about the merits of doing this, with some sources pointing out that you can simply buy a fretless neck for a reasonable price. But I certainly remember bass playing friends of mine removing the frets from basses, to convert to fretless, and I can understand why you would want to do this with a bass you don't currently use.
I'm pretty sure my friends just knocked the frets out and didn't worry too much about doing it properly… But there is plenty of info online about how to take a more measured approach.
One comment that seems to pop up regularly, is that you should be okay as long as you are not bothered about putting frets back in later! This is because you may well cause some damage to the neck while removing the frets. It's easy to fill any splinters that come away from the fretboard, then sand and refinish, but it would then be hard to refret.
I managed to find a site where you can download a free eBook taking you through the full bass-defretting process, here. I downloaded the book and it looks pretty good; it has really detailed information and a lot of pictures! It is recommended by other sources online.
A much shorter version of the same method can be found here. I won't simply copy the text here, but some of the main points are:
- remove the neck if possible, before starting this job.
- don't use a screwdriver to remove the frets (!) Instead, use "end cutters, a flat nail clipper, or another specially designed tool to lift the frets out."
- try to glue down any splinters that come off the neck immediately; this will mean you have less filling and refinishing to do later.
- if the frets don't want to come out, soften the glue by applying a soldering iron to the frets directly, but be careful not to scorch the wood of the neck.
- if you want the fret grooves to match the colour of the neck (i.e. you don't want "markers") then save sanding dust from the neck as you sand it, you can mix this with glues to make a matching filler.
- "use a radius block to sand and finish your defret"
Other sources suggest a couple of other useful points:
- put masking tape either side of the frets before removal, to help avoid marking the neck.
- you can fill the gaps with a thin veneer before sanding to make fret markers.