When we do a upgrade it basically happens in three stages.
1. Pre installation processing. Here you might be doing things like
deleting redundant information, copying old configuration files to a
safe place.
2. Copy of files.
3. Post installation processing.
If there is no record of a previous installation the first stage is
skipped.
A good example of why it matters is the kernel upgrade procedure. We are
supposed to do a rpm -ivh instead of rpm -Uvh for the kernel rpm. What
this does is it doesnt delete the old kernel. So if you look into your
/boot directory you will notice two different kernel's sitting out
there. This is needed so that you can fall back to your old kernel if
something goes wrong. Doing a Uvh would have removed the old kernel.
If your database says there is no previous installation of a application
doing a upgrade would be equivalent to a fresh installation the only
problem would be that you as a user would be under the impression that
your system remembers what it was earlier whereas it most probably isnt
so anymore.
Mithun
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