Ok, I'm doing a tune with a samba feel, the chords go

Main / Bridge
Fm9 Eb9 Db9 C7

I need a really jazzy feel, and I can't switch fast enough between F dorian and Eb mixolydian.

Thanks a lot for any help!
It's better with no teeth, trust me. Much fewer scars on my penis now that I've switched from seniors in high school to senior citizens.
You really don't have to *switch*... you could simply emphasize the new root note over the chord as it comes in, along with the chord tones to emphasize the *switch* - Don't think of it as changing positions physically on the fretboard - try imagining the chords coming up, *hear* them before they are so you're ready to *switch* as they change.

May I prescribe you with a dose of Melodic Control courtesy of Marty Friedman.
You could use F Aeolian and Eb mixolydian over the respective chords: same notes in the scale, different notes emphasized.

However, as johnljones said, it's not really switching scales that you should be thinking about: you should be envisioning the upcoming chord. Of particular importance are your thirds and sevenths, and then extensions: the more interesting harmonic choices, but ones that also fit well.

Let's take your main theme: Fm9 has Ab as a third, Eb as a seventh, and G as the ninth. F is the root and C is the fifth. Eb9 has G as the third, Db as the seventh, and F as the ninth, with Eb as the root and Bb as the fifth.

Now, what are your options? The seventh of Fm9 is the root of Eb9, so if you must you can just sit on that note. It's not the most interesting choice, however; there's no movement over the chords. The same for the ninth of Eb, which is the root F of the previous chord.

Much more interesting choices lie in the half step movement from the fifth of Fm9 (C) to the seventh of E9 (Db). Really emphasizing those notes as you switch chords will sound like you're playing the changes. Even better is the switch from Ab on the Fm9 to G on the E9; a half-step movement from one third to the next.

These two examples work great because they are half-step movement; there are also several whole step changes you could make over the chords, and by all means they can be used, but half step movement emphasizes tension and resolution much more, at least to me.

Once you've figured out a plan for moving between chords, you can start mixing in your scales as you play over the individual chords, as long as you keep in mind where the changes are coming up. Be careful about starting on the root, as it can sound like you're just practicing scales. Try starting on thirds and sevenths and mixing it up.

This is just the basics as far as soloing over jazz changes: eventually, you might want to use more "outside" notes for even more tension and resolution... however, one step at a time
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