Are you going to move both mailboxes and Lync accounts for the users to another Office 365 tenant? Staged migration is not support between O365 tenants. To migrate mailboxes from tenant A to tenant B, you can consider creating user accounts in tenant B using the default domain.onmicrosoft.com first, then do an IMAP migration. After the mailbox data is migrated, delete domain B from tenant A, verifed it in tenant B, change DNS records, change user accounts from using domain.onmicrosoft.com to domain B.
If there are few users need to be moved, you can also use .pst file to move mailbox data
