Upgrading from Debian 8 (Jessie) to Debian 9 (Stretch) will retain the old interface names (eth0, eth1). This was intentionally done to provide a path of least disruption when it came to upgrades. New installations will automatically use the new naming convention.
If you’d rather migrate to the new interface names, remove the following file:
/etc/systemd/network/99-default.link
Then edit /etc/network/interfaces accordingly with the new interface name.