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Can I swap that reply for a "why don't you use the search function...Heartthrob with two Tees would search a bit better?
And show/link what you are talking about.
https://postimg.org/image/abiztiq3v/ The rate is set with fixed parts. In a scheme like this, there's no "easy" way to make fixed part vary slowly from one value to another.
Similar to the problem of making a semitone glissando on a highly fretted instrument. Easy on violin, rough on frets/finger guitar, "impossible" on piano.
(OK, slide-guitar glisses fine, and I have worked with pianists who applied a tuning-wrench in performance, much like some banjo-players twist their tuners for emphasis.) An additional, personal, problem is that I do not see how that oscillator works. It seems to have two very different paths and modes for slow and fast. Maybe I'm not looking at it right.
If we can identify a single resistor with a large influence on rate, without much effect to anything else (loop gain, limiting amplitude), we could replace it with a photo-resistor, an LED, and a current source rigged to slide slowly from, say, 0.5mA to 2mA, and back. Photoresistors are getting hard to get. There may be no good way to control rate with one resistor. (Two is possible but tracking may more than double your headaches.)
"THE" way is to scrap the semi-fixed oscillator and build a voltage control oscillator. Switch two control voltages at it, through a filter to smooth the transition. There's a bajillion VCO plans. The problem is finding one with good wave-shape AND output level and bias which suits this crude amplitude controller (a near-naked BJT).
At this point you start to think about a "better" VCA (voltage controller attenuator/amplifier), to simplify interfacing with the VCO (and maybe gain signal quality). Now you've thrown-out the whole baby and the bathwater.
So... no. Keep this one for what it is and explore other trem plans for the sound you want.
