I’m not sure what was done or what the actual failure was, but if the “LINUX” key/value pair in “/boot/extlinux/extlinux.conf” names a kernel Image file, then that is used (except if security fuses are burned, in which case only a signed partition is used). Also, did something actually fail in your test, or did the kernel you want to run just not run? If there was an actual failure to boot, then it is evidence that the kernel was indeed replaced successfully. The question then becomes one of if the release version was correct, and if it was configured correctly before install.