Newbie to Linux: Any easy way to migrate user changes when re-installing?
 

I have considerable experience with all sorts of operating systems over many years, but am a total newbie to Linux. As a learning exercise I have installed two Slackware 13.1 machines, and am experimenting with their setup and interaction with a fair degree of success. One thing concerns me. If I tailor a Linux system to my own liking, how difficult will it be to install the next version of Slackware without wrecking all my customisations? Are there procedures to "backup" user mods? (For example the startup scripts). Any comment on the suitability of Slackware as a learning exercise (I hate installs that do everything for me, because I never get to understand how they work!).

Yes, just include them in your backup plan. If you later have to re-install just restore them.
When it comes to custom applications, that are not part of the standard Slackware installation you may have to re-install/re-compile if the underlying libraries have changed.

In Slackware, system customisations are all done under the /etc directory. As is suggested in the Slackware upgrade documentation, you can copy this directory to a safe location prior to an upgrade so that you can refer to it.
If you use the 'slackpkg' tool to conduct an upgrade, you will (by default) be prompted if a new configuration file is different from an existing file. You can choose to view a diff between the old and new file and then either i)keep the new file to consider later, ii) immediately overwrite the old file with the new file or iii) delete the new file immediately.

User customisations will be done in the user's home directory. These are generally not touched during an upgrade.

Only if you use a separate /home partition.

As well as copying any files I've altered in /usr or /etc, I always keep a file with a list of
1. any alterations I've made to configuration files outside /home
2. any software I've added from the repository
3. any software I've compiled, with download addresses and details of dependencies.
It makes the job of setting up a new version so much quicker and avoids those moments when you look for a program only to find you haven't re-installed it.

Also, back up any configuration files or directories in your home directory that you have customized or that contain data you wish to keep. For example, I back up my .opera folder, my .pan2 folder, and my .fluxbox folder, among others.

Then, for example, when I reinstall a system or install to a new computer, I install Opera, then

1. make sure it's working,
2. shut it down,
3. rename the default .opera file to .opera.orig or some such thing (I don't delete it because, if something doesn't work, I want it for trouble-shooting), and
4. import my backed-up .opera directory.

If everything goes right (and it always has knock wood), all my settings, my mail store, RSS feeds, my newsgroups, and all that other stuff has been restored when I restart Opera.