For sites transitioning from Page Manager to Composer, we are able to offer a spreadsheet to help the transition process by redirecting or deactivating all the old pages in bulk at the time of launch. This process is usually started in deployment by the Project Manager and carried out by the Support Team member in charge of launching the site through a Support ticket.
The spreadsheet is a Google Sheet listing the most popular pages on the site according to Google Analytics and includes useful information like the title, page type, keyword, statistics (number of hits over the past year and percentage of overall traffic), as well as direct links to the pages in the Published, Unpublished, and Page Manager views. Simply fill out the full URL of the page you'd like to redirect to in Column A. Column headers are tagged with tooltips with further information and instructions; just hover over any header with a black triangle in the corner for details.
We can use the spreadsheet to bulk update those pages. The rest of the pages are marked "deactivated" (more about redirects and deactivated pages can be found in these Knowledge Base articles here: Deactivate Page Manager Pages & Redirect Page Manager pages to Composer). Converting these pages to redirect is actually changing the page type to "Redirect", so with Portal and Multi Content pages, the elements on the page will be removed in the process; you may want to make a clone of the pages beforehand.
Redirecting will transfer all of Google's knowledge (aka: "link juice") of the old pages to the new pages, so they retain their search rankings. Google may take several days (in some cases several weeks) to remove or de-index the old pages from public search results, so this can also help user who stumble upon an old link reach the new version of the page.
Deactivating pages will also help Google de-index them from public search results, however they return a 404 "page not found" page to the user. This is a natural and perfectly normal part of the website transition process for pages that need to be essentially deleted from the public view.
In both cases, you're still have access to the old pages in the Page Manager and Unpublished views if you need them. You can also let us know if there are any pages you don't want deactivated or redirected. This is totally optional and any questions or concerns can be addressed in the Support Ticket associated with the process.