I started on guitar in 1978, but didn't get round to bass till 2000. Although there were some transferable skills (fretting, plectrum playing, fretboard note knowledge), bass is a very different instrument to guitar, and needs a different approach. It was about 3 years before I got anywhere playing bass fingerstyle, and 2 years ago I completely re-thought my right-hand technique.
It was a massive advantage to be able to read music (feel free to argue about this in another thread!), and I have always transcribed guitar and bass parts from recordings, so that gave me a stock of sheet music to work from. I got into playing bass in bands pretty soon, so that gave me the impetus to learn entire bass parts. At the start it was rewarding to work out and play a bass line, and thinking about it, it still is!