The next version of Windows Phone is expected to be released in the spring of 2014. Microsoft plans to make all WP8 devices backwards compatible with whatever the latest version of Windows Phone will be.
This is not going to be another Windows Phone 7 devices not being able to update to the latest version. Every phone from the Nokia Lumia 520 to the top of the line flagship devices are supposed to get the update when it comes out in the spring of 2014.
There were concerns at one point that the Windows Phone 8 devices would be cut short of Microsofts latest and greatest thing when it surfaces, but we can now breathe easy.
Source: Computer World