Dealing with a massive product that is already on a market with active user base has its peculiarities.
1️⃣⠀The scope. I needed everyone on the team to be on the same page on design objectives, priorities, and the acceptance criteria.
2️⃣⠀User research. Research existing users of each type and find what they use, what they don't use, what is giving them a hard time, etc.
3️⃣⠀A transition strategy. A plan how to introduce changes gradually without giving the app that 'Frankenstein' feel along the way.
For me, good communication has become the key to success. Stakeholder interviews, requirement workshops, and regular team meetings have tremendously helped to make re-design scope and purpose clear to everyone.