There are three different approaches to upgrading an existing WSS 2.0 or SharePoint Portal Server 2003 installation to WSS 3.0 or MOSS 2007, which can be summarized as follows:
For this type of installation
Use this recommended approach
Small deployment with few customizations
Large deployment with customizations
Side-by-side (gradual) upgrade
Farm deployment with hardware upgrade
Table A-1 lists the differences between each approach.
Sites are replaced with new version.
Both versions of SharePoint run on the server and site collections are moved one at a time.
Content databases are copied and then upgraded in place.
Before you upgrade, complete these tasks:
Review your hardware to verify that there is enough disk space for the type of upgrade you are planning.
Review your existing SharePoint installation to verify that it is up-to-date.
Run the pre-upgrade scan tool (PRESCAN.EXE) to find custom site definitions and customized (unghosted) pages.
Perform a trial upgrade of one or more site collections.