I use iTunes, I usually just upgrade over the old program unless the upgrade is to a new revision number (ie: version 4.0 to version 4.1, I will just install over the old software. Version 4.x to version 5.0 I will uninstall the old software first). The reasoning being that a new revision usually incorporates some major changes so I want as clean an install as possible. Either way you choose to do this update will work fine.
Your iTunes user information is contained in the two files (iTunes Music Library and iTunes Library) that are inside the iTunes folder in My Music. If you uninstall the old version these files along with your music remain in this folder and when you reinstall iTunes will find this information and just reload your old account.
I have never had a problem with losing music by uninstalling the old software but to be on the safe side I would recommend that you copy your music files to a backup disk to protect you in case something ever happens to your computer.
Tip: when you backup your music files, save the two files mentioned above also and you will not have to waste any time setting iTunes up again if you ever move to a new computer or restore your computer from a backup disk. Just copy your music and these two files back into the iTunes folders in My Music and everything will be like it was before.
Here is a link to a PDF file that explains how to use your iTunes music library to greater effect;http://www.phptr.com/content/images/013148...454_pp79-86.pdf
There is no justice, there is just us.