Well I dont know how many pro's are in here, what kind of music do you play????
I say listen to bass parts in songs, learn songs, and jam with other people first.
Quit the guitar mindset immideatly, start working regular with a metronome.
Play with your fingers at first, so you could have it both ways.
Get a teacher.
When jamming follow the drums, not the guitar.
Syncopated notes are awesome.
For long you live and high you fly
But only if you ride the tide
And balanced on the biggest wave
You race toward an early grave.

Ben Hamelech
Metronomes are your friend. Learn to slide into the "Groove" with the drummer, if you have that and nothing more, you will still be a good bassist. Learn scales galore. They will transform your basslines from root note mania, to an awesome groovy bass line. Play with your fingers! I always use a pick on guitar, but when it comes to bass, I always use my fingers. You can get into with your fingers. If fingers don't work, go back to a pick, no big deal, but always try it out. Also look into slapping and tapping when you've progressed furher down the line in your playing.

Read the Faq posted at the top of the forum, it'll really help you out with taking care of your stuff, and has some nice informative tid-bits in it over bass. As for the guitar->Bass transformation, remeber that your job is to lay down the low end now. If you played rhythm guitar, then you should already understand everything that the bass does. I suggest getting some books or DVD's over how to play so you can learn technique that way. Or get a teacher.

Good luck with your playing!