Inspired from a Motherboard modding -- Replacing frequency generators I'm writing about my experience with one of my 486 boards.
I wanted to check whether a 486 board is able to change clock frequency when it is fully operating without crashing the system.

It worked with my ASUS PVI-486SP3.
It has an external VT8228 clock generator.
I have mounted a 3 contact turbo switch on JP26.
It allows to change the clock frequency on the fly from 40 MHz to 25 MHz when the board is running without doing a reset or power cycle and without a crash.
It seems like the VT8228 is changing the clock so soft or slow that even a running 4x clocked AMD 5x86 133 can be switched from 160 MHz (overclocked) to 100 MHz and vice versa.

Have you ever experienced this behaviour in one of your motherboards?
(When you try this, do it at your own risk.)