A website redesign often means a complete rebuild and migrating content from the old site.
When it's done well, the new site replaces the old one and everyone loves the changes. When it's not, people are upset that things are missing or not working properly.
Scope
These projects often begin with a discussion on why they need a new site. Maybe they started with a site builder or a static site and want to have their own CMS. Maybe they just want a redesign and the current platform is fine. Maybe their previous developer used an obscure platform and the current one wants to replace it. These conversations bring out the pain points to resolve, and objectives of the new site. Follow up with discussions on requirements and scope to fit the budget, along with their appetite for follow up work.
It's important to understand the needs of stakeholders, and set expectations when scope is to be limited. This needs to be done by the organization, deciding what is low priority and can be cut or delayed to a later phase. It is better to assess this upfront than to reveal the project at launch and find out that people had requirements that were not met.
Content Audit
Begin by analyzing the current site's content to understand the model and how it maps to the new site.
Create a spreadsheet, with each page documenting the content entitites and fields. Check how images and files are stored. Check for html and classes in text content that you will have to clean up or support.
Determine who is accountable for types of content on the site. Work with them to understand the content and how it should be imported.
Architecture
Identify the parts of the structure that are solid, and where changes are needed. Start with the major content types for the site and add new entities to handle relationships or sub components.
Begin building the new site to verify the new content model, while migrating existing content into these fields.
Review
Give stakeholders a way to review the content migration before going live to check for issues.
Create a screen with links to the new and old content for easy comparison.