As a general rule, there are no additional requirements to use Debian 9/Stretch LTS. Control of the security rep is handed from the Debian Security team, to the Debian LTS team, so on your end (at least initially) nothing changes.
However, there are some limitations.
One is backports. The LTS team do not support the backports repo - at all. Generally the backports repo will be archived soon after Stretch is passed to the LTS team. My guess is that it's fairly unlikely that this will apply to you, but if you are using backports, no further software updates will occur. Once the repo is archvied, you will see errors when you run 'apt update'.
Another factor to consider is that some packages will not receive LTS/security updates. Again, my guess is that this probably won't apply to you, at least not initially. But it's wise to install the debian-security-support package. That will check for packages that are no longer supported whenever you install updates. It also provides a "check-support-status" command which allows you to manually check for installed packages which are no longer supported. Note that as time goes on, support for more packages will likely be dropped.
Nothing required to use LTS
As a general rule, there are no additional requirements to use Debian 9/Stretch LTS. Control of the security rep is handed from the Debian Security team, to the Debian LTS team, so on your end (at least initially) nothing changes.
However, there are some limitations.
One is backports. The LTS team do not support the backports repo - at all. Generally the backports repo will be archived soon after Stretch is passed to the LTS team. My guess is that it's fairly unlikely that this will apply to you, but if you are using backports, no further software updates will occur. Once the repo is archvied, you will see errors when you run 'apt update'.
Another factor to consider is that some packages will not receive LTS/security updates. Again, my guess is that this probably won't apply to you, at least not initially. But it's wise to install the debian-security-support package. That will check for packages that are no longer supported whenever you install updates. It also provides a "check-support-status" command which allows you to manually check for installed packages which are no longer supported. Note that as time goes on, support for more packages will likely be dropped.