As early as possible gather together the teams that will have an impact on the final site launch. That may include project managers, design team, development team, user experience, copywriters, account executives, network administrators and possibly even legal if they need to give their approval.
Write on the whiteboard what the problems the new website will be solving. You are planning the migration for a reason, make sure everyone is aware of those reasons and understands why they are important enough to burn the resources to make all this happen. Knowing that the old site struggled with time-on-site or click-through rates will help the teams focus on making those better on the new site.
Plan the terribly expensive meeting for everyone to sit in one room at one time for no more than 1 hour. In that time everyone needs to offer up their concerns. No one can say “I have no concerns” because if they do, they’re lying.
Write all the hurdles down so the next time you meet with any members of the team you can address their concerns. A site migration is a big deal; you’ll need buy-in from the whole team.