If we did this properly, then we’d need to
Collect requirements/desires as we examine the codebase.
Review the existing codebase, figuring out what we want to change
Determine if an overhaul vs rewrite is more appropriate, with long term maintainability and reliability and performance in mind.
Create a rough conceptual design
Setup source control and a prototype-dev-beta server (maybe even a task-issue-manager/bugzilla),
Iterate with some sort of sprint style, where we’d prototype/refine, and have a predefined target for switching the website over (upon approval from the mods/admin).
A key question would be whether to combine databases, such that a story could be cross-listed, BUT also maintain backward compatibility with existing URLs. Categories would remain, but could be implemented as, the original poster mentioned, tags, or another mechanism.