To ensure a smooth transition from a legacy document management system to an Azure AI Document Intelligence solution, you should first analyze the existing document types and data extraction requirements. This will help you determine if a custom document intelligence model is needed. Once you have designed the model, you can then migrate the documents to Azure Storage and start using the Document Intelligence solution. Option A is incorrect because simply migrating the documents and using prebuilt models may not meet all the data extraction requirements. Option C is incorrect because creating a new Azure AI Search service is not necessary if the existing system can be integrated with Azure AI Document Intelligence. Option D is incorrect because using Azure Machine Learning to build a custom machine learning model is not the recommended approach for implementing a document intelligence solution.