Upgrading from Office 2010 Pro to 365 Pro Plus - I know you must uninstall 2010 and install 365 or can run them concurrently. Question is - if you uninstall 2010 and install 365, what settings are lost and what settings are retained? Outlook signatures?  Word and Excel settings and preferences? 

Also must the OST files be re-downloaded again?  I hear there's a 40% compression improvement so this might be ideal, just want to know if the GPO to not re-download the OSTs actually works in this scenario?

Hi,

Uninstall Office 2010 will not remove any user settings, the user settings will migrate upon the first use of each Office 2013 application.

By default, when Outlook 2013 is installed, a new compressed version of the Outlook data file (.ost) is created. This new compressed version of the .ost is up to 40% smaller than the size of the .ost files that were created in earlier versions of Outlook. So it is recommended to use the new .ost file.

The "Do not create new OST file on upgrade" policy also works for Outlook 2013, however you might need to download the new Outlook Group Policy template (Outlk15.admx). Outlook 2010 template should not work in this scenario.

Regards,

A.B.