This article explains a system upgrade procedure from Debian 8 Jessie Linux to Debian 9 Stretch.
Apart from the up to date Linux kernel, Stretch comes with a considerable amount of new and updated software as well as a number of packages had been rendered obsolete:
This new release of Debian again comes with a lot more software than its predecessor jessie; the distribution includes over 15346 new packages, for a total of over 51687 packages. Most of the software in the distribution has been updated: over 29859 software packages (this is 57% of all packages in jessie). Also, a significant number of packages (over 6739, 13% of the packages in jessie) have for various reasons been removed from the distribution.