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How to add a trial period for an active subscription?
The moment a subscription moves from the 'In-Trial' to 'Active' status, it can no longer be reverted to the In-Trial status. The only time when it is possible to add/extend a trial is while the subscription is still in the In-Trial status.
If you still wish to offer a trial period you would have to either,
a. Create a new subscription for the user, so you can offer them a trial period
b. Offer a coupon for the amount= price of the first month; so it can be considered a trial period.