I also started on fretted and branched out to fretless later on. The major change in technique I had to implement was to fret at the fret lines. This was a marked change from playing fretted where the frets saved me from sloppy fretting. One thing though is that when I went back to fretted after going fretless for a while, I noticed that my fretting "cleaned up". I guess this was due to fretless playing being more precise. Of course, all that I've said is premised on "fretless" being "lined fretless". If your concept of fretless is unlined, I'm with Jerry as far as letting your ears do the playing. I guess a good source for tips would be double bass players. Regardless, your ears play a major role in fretless playing. One thing for sure, you'll pick-up a lot of fretting hand techniques along the way to make the bass produce a wide palette of sounds you wouldn't be able to do on a fretted bass, most of which is through trial-and-error/experimentation.
Stepping aside, I'm of the opinion that my playing would have been better if I had started off playing bass on the fretless/lined fretless foot rather than on the fretted foot.