For sites with hundreds of pages, a redesign is a daunting task.
Let's break down how to think about this:
1. 80/20 rule
Chances are that the majority of your content out there is for SEO (or knowledge base). These pages don’t need major design work, they just need to pick up the new styles.
Focus your effort on the key marketing pages where the story needs to be told better with new design elements. Use the Pareto Principle: pick your 10 most important pages first. Force yourself to clarify the story there.
Then, expand to the top 20% of pages, making sure they fit into the hierarchy you established with the initial 10.
2. Instill hierarchy
I've seen a lot of sites with thousands of pages and all of them are at the "root level" (domain.com/really-unique-page-goes-here).
When going through this exercise, make sure you place pages underneath the page they belong to.
Without hierarchy, you simply have an alphabetical list of pages in your CMS and it becomes hard to keep it clean over time.
3. Batch the 80%
For the perfectionists out there, this is the hardest. We've defined the 20% of pages that need manual labor. Don't fall into the temptation of perfection for the remaining 80%.
Chances are high that they are simply walls of content and the newly built website will apply updated styles to them and they will look fine.
Yes, someone should do a quick glance over them to make sure nothing atrocious has happened during the migration, however, don't spend an inordinate amount of time here.