Even though it’s easy as far as the technical side is concerned, a bass has a completely different logic to it. It obviously depends on what you define as “playing”. If you want to just hit E,E,E,E,E and then E for half the song and then go up to B for a couple of bars, then any fool can do that, even if they’ve never seen a guitar before. But proper bass playing means two things:
1) a decent knowledge of music theory and harmony, ie always knowing the chords and all the notes that correspond to them, as well as those notes that will help you either improvise a harmony on the spot, or come up with certain patterns depending on the style and song (you could simply find these in a book, the point is to be able to understand their logic).
2) synchronisation with the drummer (I’m thinking of your typical drums-base-chords-solo band here, where the solo instrument is usually an electric guitar, and the chords are played by an acoustic guitarist, who is usually also the singer). Synchronisation in turn implies first of all some basic talent (I have none of that) and secondly several hours rehearsing, especially with the drummer, and preferably NOT under the influence of alcohol, which is how most musicians rehearse.
So, even though knowing how to play the guitar can help (and if you’ve played the classical guitar for many years you’ll certainly know about base lines and so on), in most cases all you’ll know is how to hold the bass properly and maybe how to tune it and that’s all. To become a decent bass player you’ll need to apply just as much effort as any other instrument.