Do former members get another free trial?.

Regardless of what the user does, they will not receive a new trial period besides the one they already got. If they upgrade or you have assigned them a new plan, they will not get a trial, but they will pay according to the payment schedule for that subscription. 

However, if users are on a free trial and then switch plans, they will keep the trial they were in, even if the new subscription does not have one. For example, if you have plan A, which has 14 days of trial, but on day 2, the user changes to plan B (that might or not have a trial), they will still have the next 12 days of the free trial. After that trial s over, they will never have a free trial again. Rest assured, your users will never be able to hop from trial to trial to have free access to your content.