Is there an easy way to upgrade the kernel but leaving the other installed stuff untouched? I have the the 18.104.22.168 kernel from the SheevaInstaller, installed on the NAND.
Some of the "other installed stuff" may have dependencies on functionality and/or interfaces in your specific kernel version. Upgrading the kernel without upgrading dependent drivers and/or libraries may cause things to break.
Having made that disclaimer, 2.6.30 (your current kernel) is not far from 2.6.32 (the kernel that supports your device) number-wise and may
not have a large change in interfaces. I don't know for certain, though. ie. Keep a copy of your current kernel just in case you need to go back after updating.
As for upgrading the kernel, the Plug Wiki
may have some entries for that. The basic steps are: find/get the kernel you want to use, erase the area of NAND where the current kernel lives and copy the new kernel into that NAND area. You can use u-boot to do these last two steps.
As long as you don't erase u-boot, you should be able to recover by using the same steps to put your current kernel back into the NAND.
EDIT: added link to pre-made kernels and modules