Upgrade vs. Clean Install
Leopard offers a choice of four install options: install, upgrade, archive and install, and erase and install.
Install is the option selected when no Mac OS X version is installed on the target hard drive or partition. If you have already erased or partitioned your hard drive or are installing onto a new hard drive, this will be your only option. The other options all relate to installing on a hard drive or partition in which an earlier Mac OS X version is present.
Upgrade, as it its name implies, updates the existing Mac OS X installation. Upgrading is the easiest method to install Leopard. It preserves the existing user accounts (along with all their settings and files), most system settings, and doesn’t require any setup tasks once the install is complete.
Upgrading is the most tempting choice because of its ease, but it may not be the best option. Upgrading will selectively replace Mac OS X and application files based on those that have changed. If any files are corrupted, have incorrect permissions, or have been modified (either by you or by software you’ve installed), there can be problems after Leopard is installed. Upgrading can also leave behind extra and unwanted files, resulting in decreased disk space.
Another consideration is that all previous Mac OS X versions stored user accounts and related data in a NetInfo database.
With Leopard, Apple has replaced NetInfo using a new local LDAP node to store this information. This is good news for under-the-hood performance, but if there is anything unexpected in your NetInfo database, there could be problems converting those accounts to the new format.
If you are experiencing any issues with your Mac or any applications, you are probably better off doing a clean install than an upgrade.
Also, if your Mac has had successive upgrades from even earlier Mac OS X releases, you are better off with a clean install because there might still be stray files and potential issues from those previous upgrades.
Even if this is your first Mac OS X upgrade and you are experiencing no problems at all, a clean install might be a better choice simply because it ensures that everything is clean and entirely Leopard.
Doing a clean install also performs a sort of spring cleaning for your Mac. Often, applications and installers will deposit additional files into Mac OS X’s system folders.
While this isn’t always bad, if you remove applications, these files typically remain, taking up valuable space. In some cases, there may even be leftover processes that get started when you turn on your Mac from previous applications, taking away other system resources. Doing a clean install removes these files and processes.