It's possible that the switch from RHEL 4.0 to Red Hat Linux 8.0 also involved a switch from XFree86 to Xorg. If that is indeed the case, then the keycode for F12 may have changed.
From the Arch Linux Wiki:
The most global way to diagnose the situation is to go in console and use the 'showkey' utility. Please note that this utility will not work when in X so you'll need to switch to another tty (e.g. CTRL+ALT+F6).
First of all we run:
without options: 'showkey' waits for ten seconds that we press a key (after ten seconds it quits automatically) and displays the keycode of the key we pressed.
So, to learn the correct keycode for F12 in Xorg, switch to a VT and run 'showkey'. You have 10 seconds before showkey will timeout. In those 10 seconds, press the F12 key. The return will be the keycode for F12.
Then, edit your xmodmap
keycode 96 = XF86_Switch_VT_1
to show the keycode for Xorg.
In my SuSE installation, that keycode is 88.