Build the redirect map before the migration, not after

Build the redirect map before the migration, not after
Migrations fail in one specific way: somebody owns the design and somebody owns the platform, and nobody owns the addresses until launch week.
Do this first, while there is still time to argue:
— Export every URL the old site has. From a crawl, from the sitemap, from the logs, from your analytics tool. Four sources, because each one misses something different.
— Deduplicate, then mark each address: keep, merge, or retire.
— Map every kept and merged address to exactly one destination. One to one wherever possible.
— Never map everything to the homepage. That is a graceful way of saying you gave up.
— For retired addresses with no equivalent, decide deliberately whether they simply go missing or are declared gone.
— Test the map against staging before launch, not after.
— Keep the old logs. Afterwards they are the only way to find what you forgot.
The map is the migration. The rest is decoration.