What are the best practices for migrating from an old document management system to a new ECM?

[Asna08]

We are planning to migrate from our legacy in-house document management system to a modern Enterprise Content Management (ECM) platform, and we're currently in the strategy and planning phase. Our goal is to ensure a smooth transition with minimal disruption, while also improving our data governance and search capabilities.
We're looking for insights from the community on a few key areas:

1. Data & Metadata Integrity: What are the most effective strategies for ensuring all documents, associated metadata, and version histories are transferred accurately?
2. Phased vs. "Big Bang" Approach: Has anyone had success with a phased migration (e.g., department by department) versus migrating everything over a single weekend? What are the pros and cons you experienced?
3. Tools & Automation: Are there any recommended third-party tools or scripts that can help automate the process of data extraction, transformation, and loading (ETL) between the two systems?
4. Change Management & User Adoption: How did you handle user training and manage the transition to ensure employees were comfortable and proficient with the new ECM?