There’s no switch. It’s a continuously variable system. Even with lactate tests, a range is identified not a tipping point. Also, you don’t have to ride at “fatmax” to improve “fatmax”. You just need to do more endurance paced riding. It’s really simple and doesn’t need to be complicated with lactate.
It’s really easy to test one’s FTP and that’s free. For the lower point, I just ride in my WKO5 derived zone2. ISM’s 2mmol lactate zone2 is probably closer to tempo anyway. Ok, so ride some tempo if that is in your plan. I just can’t see paying for a meter and the strips to figure what is already easily figured out. And FWIW, since ISM is really vague on training prescriptions I wouldn’t follow anything he says. The only message I get from his is to ride more endurance. Ok, check - I already got that down.
If one wanted to monitor these kinds of levels daily and/or in real time a Moxy (NIRS muscle oxygen sensor) would be the way to go. Some coaches have completely switched from lactate testing to NIRS because it’s a one time cost and the athlete can use it every day.