Microsoft has recently updated its document Migrate to Microsoft Online Services.
“Microsoft® Online Services supports migration of the contents of local Microsoft Exchange Server mailboxes as well as the contents of POP3 and IMAP4 mailboxes from your current environment to Microsoft Exchange Online. Migration can be done as quickly or as slowly as you want. Small organizations may be able to migrate to Microsoft Online Services overnight or over a weekend. Larger or more complex organizations typically prefer to establish e-mail and directory coexistence for a longer period of time and perform a controlled migration in logical stages.”
This document describes how to migrate your on-premise Exchange Server mailboxes and POP3 and IMAP4 mailboxes to Exchange Online.