Migrating Lotus Notes into Exchange

Hi there,
I have Two client one with windows nt 4.0 users and Lotus notes R5 as email system.and the other with Windows 2000 Active directory forest and Exchange 2000 as the email system.

Now here’s the thing.
This two clients want’s to become one company and migrate all the user account and mail boxes into one single new domain. With Windows 2000 ADvacnce server and Exchange 2000 Server.

My concern is how to migrate this users and mail boxes into a new single domain.
and How much willbe the impact on users if we do some solution.

More details if requested.