GNU/Linux Desktop Survival Guide
by Graham Williams
Debian provides a very simple approach to upgrading (or downgrading) kernels. Kernels are simply packaged as Debian packages and installed in the usual way (see Section 6.6.3).
Note that Debian treats kernel versions as different packages. Thus, installing kernel-image-2.4.9-1-686-smp installs a different package to kernel-image-2.4.6-1-686-smp. You can have multiple kernels installed at any time, however only one kernel is ever chosen at boot time (and you can have the choice, although a default will be chosen each boot). The advantage of this is that you can be confident to install a new (perhaps experimental) version of a kernel and reboot into that version, but always be assured that you can go back to a previous version if there are problems with the newer version. Old versions hang around until you explicitly remove them.
It should also be noted that within specific package versions (e.g., kernel-image-2.4.9-1-686-smp) there are also normal Debian package versions (e.g., 2.6.9-2). Potentially, you may be installing an update of a currently running kernel, as in updating kernel-image-2.4.9-1-686-smp from version 2.6.9-1 to 2.6.9-2. Some complications can arise when installing an update of the current version of a kernel. A message may appear indicating that you are upgrading the current kernel and the modules will be overwritten. As the message will indicate, you should reboot your computer as soon as possible, since these modules are probably currently being used by the kernel!