What about config Files during upgrade ?
 

During upgrade the system ask me what to do about config files.
K to keep old ones
then other options to change config files to the new system and backup old one with *.old extention and 2 other options I don't remember.

What do you think is the best choice if I have changed my xorg.conf, my layout and my fonts.

;) NjB ;)

During installation, I feel it's safest to keep them.

New config files appear as something.conf.new, or the like

You can check the diff (you're probably using slackpg for updates, right?). If there are significant changes (like addition of new environment variables etc) you can have a more careful look and merge the two. But this is rarely needed, especially the security updates in stable do not bring any new features, so usually it is safe to keep your modified copy and delete the *.new file.