Hi Paul,
So there are some users that already signup-ed and you are trying to add a new feature called free trial that the already signup-ed users should get?
You could create a true/false property and add a conditional link action that go only if that true/false property > is false. Or you could create a date property add the condition on the action as only when date property > is equal to > empty.
Did I got it correctly?
Thank you