Until now the 14-day trial was a flag the checkout request carried, and the API honored it. An org that cancelled and checked out again could start a second trial.
The API decides now. It refuses the trial when trials are switched off, when the org has already started one, or when the org already holds a subscription. The first trial’s start date is stamped on the org from Stripe’s own record, once, and never overwritten. Checkout still opens in each of those cases; it opens the paid plan.
The trial itself is unchanged: 14 days, every Pro feature, a payment method at checkout, nothing charged until day 15.