Once a subscription moves from a trial state to an active state, it cannot be moved back to trial organically. This is true even if you switch the subscription to another plan that has a higher trial period. The status would remain "Active".
However, if you'd like to manually change the existing subscriptions' status to In- Trial, you'd have to cancel and reactivate the subscription all over again.
While reactivating the subscription, you have an option to mention the trial end date. Once you populate the field, it'll ensure that the subscription gets into the trial state once again.
If you have any questions regarding this, please reach out to firstname.lastname@example.org