I upgraded two machines from Linux 2.4 to Linux 2.6.

System1 was installed with the old configs present.
Letting the installation find the old configs and use the same settings worked for some programs while others misbehaved. This Linux 2.6 was unusable for 1.5 years while I fought with multiple problems at once. One of which that KDE would startup and then quit every time, while xfce4 would run. The latest problem was traced to the video driver using a graphics library installed by a different video driver, which it got because a soft link was present. There were no answers in LinuxQuestions for any of my problems.

System2 was installed to a clean partition and old configs brought over after installation and the same settings re-introduced by hand. This machine had much fewer problems which only took days to resolve. At least I knew exactly where I was changing things.

These problems were due to program changes instead of hardware changes, but it is similar in that old installation setups were involved.

It is never known how programs configure themselves to the environment when they are installed. Windows is not required. The same programmers make versions for Linux too, they even keep using environment variables. Kernel customization is the most important, but if you are going to use a huge kernel it will probably work. Then it comes down to how many programs configured themselves the first time they ran.

As a quick test, do a grep for "/dev" in your hidden directories, /etc, /usr/share, and /usr/local.

Yes :)

Not surprising you DID have problems with THAT kind of transition.
Take this fact alone, that 2.4 to 2.6 included significant environmental changes. I did it on my home desktop just for the fun of it, following the guide at kerneltrap.org, but at work I waited until it would be safe to reinstall.

...In that sense just swapping a HDD from one machine to another is a much safer way to make sure everything will work. But hey, let the guy JUST DO IT and see what happens!! So much time spent discussing what is easier to see by mere action.
Then, if everything works it's fine; if he's not satisfied, he'll have to spend some time to fine-tune. In any case, he'll HAVE TO DO IT.

jlinkels and Shadow 7 are right. The kernel, unless VERY old, will automatically recognize all hardware. Suse 11.4 is not pre-historic so the kernel will be just fine. No speed loss or other troubles like with Windows which generally behaves like OEM and must always be reinstalled when switching to a different mainboard. Not so with Linux. And if you had previously installed the whole bunch of xorg video drivers then xorg will be able to successfully bring the desktop back to life, except if you were using some unrecognized special driver. Just check the installed xorg video driver packages before you make the switch. If you're using e.g. the proprietary Nvidia driver and now have some other card in the new machine, switch down to nouveau first.

The network hardware and udev, yeah. If networking doesnt work after the switch, do as jlinkels said and remove the line in /etc/udev/rules.d/70-persistent-net.rules mentioning eth0, then reboot and the network should work.

A complete reinstall is absolutely unnecessary. No performance loss, no instability, no "dead file" leftovers. Just a couple of small xorg video drivers not used which could be uninstalled to save a few hundred KB.

It is much simplified if you use DHCP to configure local networks - that is its purpose.

Other things that can get very dependent on customization are routing tables, IP tables, NFS support (borderline - sometimes you do have to change servers).

There are other services that also require changes - sshd must get new keys, Kerberos (if used) must get a new keytab file...

Anything that depends on a specifically identified host. In the user area, that can include known hosts entries, as well as some private keys (usually doesn't though).

I can't think of any specifically GUI related though (might be some if tied to a crontab though).