The good Professor might have better insight into this topic, but allow me to use my own experience.
I started as a rock/folk guitarist and graduated into jazz. What that entailed was a willingness to learn the vocabulary of the idiom as from: scales, listening to ALL jazz instrumentalists, coping jazz guitar licks, theory, expanding my chordal understanding, transcribing, etc.
I was totally unschooled so it took me perhaps a bit longer to get to a point where I became comfortable with improv and chord melody. Right or wrong I picked Joe Pass as a mentor and studied his style exclusively for awhile, maybe too long a while. I also stumbled onto Charlie Parker and bebop, and spent a lot of time (still do) listening to that era's players and their language.
The one similarity between rock and jazz that I found was the blues of all things. I still use the 12 bar progression (albeit more complicated today) to jam with and to try new ideas out. I'm not sure what one has to STOP doing. If you're undisciplined, well that's one area to improve.
If I were to do it all over perhaps I'd seek out a teacher that I respected as a player, and begin studies with. And I'd probably stop procastinating so much. If you love what you hear in jazz, go for it. Gravitate toward others playing the same music. And don't forget to LISTEN. Sorry if this all seems like rambling. Best of luck, and check back here often to ask questions, lots of questions. Practice.
Hey! You just got here, why are you asking such hard questions?
Really, just kidding.
OK, you didn't tell us your background, or how long you have been playing, or your skill level.
For the typical rock guy, jazz is going to be way over their head. Jazz is an upper level genre, and so you have to be really solid on all of your basics and "stupid" things that might distinguish a 'guitar player' from a 'musician'. Like knowledge of chords, scales, upper fretborad areas, and reading. Yes, reading of music.....proficiently. Especially in flat keys, which is something that is very hard for many guitar players. Not all charts have just chord names and 4 slashes per measure.
Work on sight-reading and rhythm.
Listen to jazz - a lot.
Also, since jazz chords are different from their more common rock usage cousins, you have to re-learn every chord you thought you knew. There are 37 ways to play a G7 chord. ;D (not all of them are practical)
Chord melody is just as hard as advanced classical guitar.
Find a college trained jazz player and take lessons.
Try to join a big band rhythm section.
You'll love it! It looks just like a Telefunken U-47.
There are a couple of ways to do it. One is to learn some jazz standards, listen to how different people play them, then add the songs to your sets. You'll be playing jazz done your own way.
Another is to take songs you already know, sub jazz chords for standards (this takes some trial and error), invite a sax player to sit in, see what happens.
I remember one of Miles Davis' interviews where he said he was looking for a guitar player who could play like Jimi Hendrix. Putting stronger, fuller, more expressive guitar solos into jazz arrangements is a good thing. Don't be intimidated by the complex progressions- what you need to do when you play with jazzers is follow what they're doing, make sure that what you play is a contribution.
I began in rock, played a lot of blues, and took 3 years of classical lessons, and have dabbled with 'jazz' !
While I can improvise blues, and solo on rock, it is CHANGES that stump me. In blues and rock you can stay in the same tonal centre and work on climaxing your solos and on hitting a few big daddy notes - wailing, screaming, riffing. But this is ultimately unsatisfying. It lacks variety and genuine musicality [it's also a bit childish].
How do jazz players manage to create such a FLOW of ideas when they are playing over changes or over unusual tonalities?
Are they THINKING of the changes; are they counting the bars and sections?
I find that I play in fits and starts, and yet I listen to someone in the style of Barney Kessel etc., and can't believe the musical flow they create, with all its seemingly effortless twists and turns. How are they able to KNIT together disparate chord progressions?
Does the rock-based player need to have a major over-haul of his whole mentality?
Yes. The current teaching is that players should think of the arpeggios that make up the chords, and use those tones primarily when improvising. The danger of that is that it will not actually be improvising. It will sound canned and formulaic. The best way I have found to learn to solo is to solo. The group you play with may need to use patience at first.
You'll love it! It looks just like a Telefunken U-47.
Many players begin in Rock guitar today. They learn to improvise over basic three chord sections, modal, blues etc.,
I ask; what does a Rock player need to do if he wants to go into Jazz?
What does he have to STOP doing, and what does he have to START doing?
STOP thinking about licks, pentatonic scales etc. START using the major scale, play it in all positions, permutations etc. Then start building melodies. Learn to invent! STOP playing a lot of root notes, START emphasizing extensions (maj 7, maj9 etc) STOP playing very strictly on the beat START playing loosely after the beat