If you wish to install the OSS version over the NE version then you just run the normal install script and it performs an 'upgrade' install. I would suggest you install the same release of ZImbra over the NE version then run an upgrade to the current version 4.5.2 Take a backup of the doirectories first, just in case.
Is that what you were talking about? You will also lose the NE functionality when you perform that type of upgrade.