It is possible to upgrade to Debian 9 from Debian 8 using ssh. This tutorial shows how to upgrade from Debian 8 Jessie Linux to Debian 9 Stretch using command line. Make a backup (I am using Rsnapshot) ??? it cannot be stressed enough how important it is to make a backup of your system before you do this. Most of the actions listed in this tutorial are written with the assumption that they will be executed by the root user
Nixcraft: It is possible to upgrade to Debian 9 from Debian 8 using ssh.