There's already some really value info here. I'll add that it's important to manage expectations and emotions of your stakeholders during these types of projects. They need to know the level of involvement you'll need from them and to what extent.

Also, let them know that whatever go-live date you set, the project isn't over. They should expect to be some issues post-launch that will need addressing. Ensure them that the engineering teams won't leave them, and make sure you have time built in to support them during that time. Whatever time you think you need for that period, double it, just in case.

All the best to you as you begin your migration effort.