I created 3 products (Monthly subscription) for Revenuecat Billing app as below:

And I created 1 offering including 3 packages corresponding to the 3 products above, just like:

Then I developed a subscription page based on RevenueCat Billing's web SDK. I first subscribed to the Basic plan, and after the subscription was activated, I subscribed to the Premium plan with the same account. At this point, both subscriptions became active, and the Basic plan did not automatically cancel as expected.
So how can I achieve subscription switching so that only one subscription is active at a time?