In the ISO image, updating the Kernel will create problems with all packages related to that version of the kernel. It is like a waterfall effect. You change the kernel and any packages in your ISO that have relation with the old kernel might show problems when installing them.
In any case I also tried to upgrade the kernel, my case was 2.6.36 to 2.6.39. No luck there was no option for me at that time but for what I have checked here: https://bugs.launchpad.net/uck/+bug/664709 and here: https://answers.launchpad.net/uck/+question/96338 there has been work to accomplish this using some methods.
In all cases, the problems described in those links suggest that doing the upgrade like I mentioned will create other problems: Not booting, problems looking for packages, package managers having upgrade problems with the kernel, etc..