Say you have a ROM installed that's compatible with the stock kernel, but you also want to run a custom kernel. You can kexec the custom kernel from CWM. And, if you want to "switch back" to the stock kernel, just reboot.So when you say you just reboot to get back to the stock kernel, is it a certain way of rebooting?
But if you have a source-built ROM (CM, AOKP, etc.) installed to the /system partition then the stock kernel will be incompatible. You'll always have to kexec the source-built kernel.
That's a point of frustration for sure. If there's no other bootloader workaround found, we've discussed the idea of hijacking part of the boot process to detect if the stock kernel is being used to boot a source-built ROM, and if so, redirect it to boot recovery and automatically kexec a source-built kernel. Of course, the details on that still need to be figured out.
Anyways, what we're planning to do on the Sprint SGS3 is make the first CM releases actually (kexec) bootable from the SD card, so they won't be written to the /system partition at all. This lets both stock ROM and CM co-exist until the bugs can be worked out, and CM is worthy of being run full time.
I'd recommend that initial CM ports for the Verizon SGS3 use the same approach, at least in the beginnig.