Jerome,

You are correct. The more you use a machine, the more "seasoned" it becomes. When the application architecture changes completely yet all the library files are conditioned to the way the older OS operated, there will be all kinds of conflicts.

Just knowing the architecture of the core processes, I can tell you that in many cases the old setting will prevail on an upgrade even for features that have become obsolete.

If it ever becomes an issue for you again to upgrade a Tiger, Panther, or older version of OSX to Leopard, simply archive your files into a backup. Once you have the files backed up, install Leopard on a new disk (or resize the old OS partition to a smaller amount and install Leopard into a new partition). Then use the settings transfer wizard to load all the old settings into your new Leopard install. (If you did the partition method, just delete the old partition and expand the new Leopard one).

Oh and just a note! If you use any computer for mission critical applications (such as a show controller), always make sure you are running fresh. I have a script which I run once a month which wipes the OS, reinstalls everything, and then transfers my Catalyst settings (or any other app settings). It takes about an hour to run but I just walk away and come back to a fresh machine.