Open Outlook. In account settings, click on the Exchange email account. Uncheck “Use Cache Mode”. Click “Next”. Click “Ok” and click “Finish”. Close and re-open Outlook. After opening, go to the Import/Export area. Export to a Personal Folder File (.pst). Click on the full email name and check the box, “Include subfolders”. Browse to where you want to create the backup and create a file name for the backup file. Click “Finish” and let it run.
After the backup finishes, you can now create your new connection to Office 365 Exchange Server. After confirming a good working connection, you can begin to manually move folders and emails.
Go to Account settings and the Data Files tab. Click “Add” and browse for your backup pst file. Click close and you should see a new group of folders in the left column. You can move items from the backup folders to the new “live” Office 365 connection.