Is it possible to reload the kernel using kexec after a kernel upgrade without doing a complete reboot? The Ubuntu wiki RapidReboot page explains how to perform that using init.d commads, any fast way (i.e. a script) for doing that only on explicit request?
That method IS the fast way that does it on explicit request. First you load the new kernel with kexec, and then you reboot and the modified script launches the new kernel instead of actually rebooting. The actual launch needs done in the reboot script because you need to cleanly shutdown the system first.