How does this change impact existing unit tests that might be using callback-based syntax for Mongoose queries?

Existing unit tests that are using callback-based syntax for Mongoose queries (e.g., `MyModel.findOne({}, (err, doc) => { ... })`) will likely fail with the `mongooseerror model.findone() no longer accepts a callback` error upon upgrading Mongoose. You'll need to update your tests to match the new Promise-based API. This means using `async/await` with test runners that support it (like Jest or Mocha's `done()` callback for Promises) or explicitly using `.then().catch()` and ensuring your tests correctly handle the asynchronous nature, often by returning the Promise or using the `done()` callback if not using `async/await`.