Most electric utilities will send someone out to confirm your service feed size for free, and usually being nice guys, most of them will also be glad to look at (but not touch) the feed to your panel as well. They can tell you if the meter-to-panel wiring is OK so you'll know if you need more work there. While they are there, ask them if your licensed electrician is permitted to change out the meter base alone or if they have to cut the current to it from the supply- that can vary by utility company rules and may need to be scheduled some days ahead of time. If the service feed isn't rated for 400A you'd do better to see if you reduce the panel's loads to 200A total as the utility company will often want a big chunk of change to upgrade a service when it's underground!

Phil