It’s quite a big change, depending on how many routes are in play. Migrating to App Router is non-trivial. Thankfully, the pages directory model is still just

It’s quite a big change, depending on how many routes are in play. Migrating to App Router is non-trivial. Thankfully, the pages directory model is still just fine, so there’s no immense pressure for existing apps to migrate, and it can be done somewhat incrementally.