Q: How do you upgrade WSS v2 pages customized with FrontPage 2003 to WSS v3 (or MOSS)?
A: Unfortunately there is not an automatic upgrade process that maintains the customizations AND upgrades to the new v3 features. Since pages that remain uncustomized (ghosted) will be automatically upgraded, the best approach is to return the customized page to its original uncustomized (ghosted) state, upgrade the page to WSS v3 and then redo the customizations using SPS and master pages. Alternatively, you could force the upgrade and then reapply the customizations.
A WSS v2 page is said to be uncustomized (aka ghosted) when it is retrieved from the file system (C:\Program Files\Common Files\Microsoft Shared\web server extensions\60). If the page is customized (unghosted) with FrontPage 2003 a copy of the page is stored and retrieved from the SharePoint content database instead of the file system. During a gradual upgrade to SharePoint 2007, a customized (unghosted) page does not automatically upgrade to the new v3 functionality (site definition), although it is possible to force this to happen. Unfortunately, if you want the new v3 functionality you must reset any customized pages to the site definition. This process discards the customizations and configures the page to use the new master pages and layouts for v3. Then you can use SPD to reapply any customizations. If you have a lot of these pages, then this will not be a trivial process. One approach may be to re-ghost the page prior to upgrade and then reapply the customizations. Once the page has the new v3 features there may be less of a need for the customizations.
Tools for Re-ghosting
This tool comes with good recommendations, so you might want to check it out to see if it meets your needs.