To prolong a trial version, create a key to change the expiry date.
Specify the number of days the trial version should be extended. The day count starts when the user applies the Activation Key
The Activation Key would change the expiry date to this date in the license file.
You could check Online Validation required to ensure, that the key cannot be used twice.
The field: Number of.... must be set to 0. Otherwise the number of licenses would be increased