I'm certainly not suggesting that it happens instantaneously because of a change from low-bar to high-bar position (in one session), and I hope no one would read it that way. Stress/adaptation and incremental loading still apply.
If you still take issue: I don't have any experience. You're right. The squat mechanics do change as this (based on videos I've seen, mostly of Wolf) puts the bar higher and forces a high-bar squat with the resultant gaze-forward, more-vertical back angle, more-forward knees, more quads/less-hamstrings & adductors. I don't think adding weight to a high-bar squat to that degree is out of the question though and would be happy be proven mistaken. What did Greysteel athlete John C. squat on his first day? What is he squatting now?
What I suggest is that I'd rather one, after having reasonably exhausted his other options, high-bar squat heavier using a crutch than stagnate at a weight because of pain with a low-bar squat and ultimately not continue to train. Reading between the lines of your challenge, yeah, I'm betting there are other factors at play here though and that BuckFifty (at 28 years old) can probably get to a pain-free rack. If he can't do that via the suggestions on this board, the SS videos on the topic, or trial-and-error with respect to grip width, thumbs around, etc., then individualized coaching or a seminar is a fine next step. If that doesn't work, safety bar. If that doesn't work, leg press. If that doesn't work, road cycling and bosu ball.