Can't find existing Fedora on FC3>FC4 upgrade

[Andy Schlei]

I am trying to upgrade FC3>FC4. I have booted from the boot.iso CD (burned) 
and am installing from HTTP with the source loaded on another machine on my 
local network.

My problem is that the installer does not find any existing implementation 
of Fedora, so goes into the new install path rather than the upgrade path. I 
have successfully upgraded a FC3 machine using this same approach (the test 
machine, which is now the source for the HTTP install) and the same CD on a 
machine that is using default LVM partitioning.

I have faiirly standard partitioning using LVM.

What does Fedora do to find an existing implementation?  How can I make sure 
that it finds it?  Any way to avoid a full re-installation.

Thanks in advance.