The following are changes we implemented to upgrade our operating system from RedHat 4.2 (Biltmore) to RedHat 5.1 (Manhattan).

If you try to follow the OS upgrade instructions in the manual, the procedure will FAIL because:

The MILO images are in the wrong directory. They are in the directory MILO instead of MILO/images (N.B. this seems to have been fixed for the PowerTools edition).
The kernel on the boot image does not work. We had download the source tree for Linux 2.0.34 and compile it yourself to get a working kernel.

The command "make menuconfig" does not work for setting kernel compile options as shipped. Information on a fix can be found here.
Make a RamDisk as per the instructions in the manual. Make a MILO image disk using the the appropriate image from the MILO directory of the cdrom. Make a boot disk by copying the boot disk image and writing a hand-compiled kernel over it.
The boot kernel reads the date from the hardware clock incorrectly (20 years in the future of what it should be. In preparation for a workaround for this bug during the installation, copy /bin/date from an existing 5.1 installation (if you have one) to the floppy with the boot image.
Follow the upgrade procedure instructions in the manual.
During initial install of the system, the following can be used to get the date right for the packages installed by the RedHat package management system:

after the menu that asks you to put the CD in the computer and press "OK", use <ctrl>-<alt>-<F2> to get an alternate console.
replace the ramdisk floppy in the computer with the boot image floppy (you will need to put this floppy in later anyway).
at the console: mount /dev/fd0 /modules. modules is an empty directory that you can use as a mount point.
type: /modules/date MMDDhhmmCCYY (MMonth DDate hhour mminute CCentury YYear).
umount /modules
<ctrl>-<alt>-<F1> and continue installation