Lumia devices currently running on Windows Phone 7.5 will not get updated to Windows Phone 8 . It is due to improvements in hardware support and the change in platform architecture.
Rather these devices will get many of the new features coming with WP8 in form Windows Phone 7.8 update. Great features such as the new Start screen , three live tile sizes will be available to current crop of Lumia devices . This update will directly come to customers sidestepping carriers.