If you've recently upgraded from a pre Environment Manager 8.5 environment to 8.5 or above. When you attempt to open a configuration it will prompt you to upgrade.
Once this process is complete and you have your new configuration you would then be prompted with the following message.
If you select 'No', your configuration will open and still operate using the Legacy Trigger perfectly fine. However, you will be prompted with this message each time you open this configuration.
If you select 'Yes', you will upgrade to the Logon Sub-Triggers, upon saving this. The next time you open this configuration you will NOT see this message.
If you select 'Yes', then you revert back to the Legacy Logon via 'Manage' > Advanced Settings, save this. You will be prompted by this message.
This behaviour is by design.