No, you will not be charged after your free Premium trial.
Once the free trial period has ended your Lookout account will automatically revert to the free version. As an existing user you can continue to rely on great free features, such as Locate and Scream, for the life of the account.
If you do wish to purchase Lookout premium after your trial, you can do so by choosing any of the premium options from within the Lookout app. More information is available here.