Are upgraded customers considered returning subscribers?

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To answer this question, we'll first define a returning subscriber (under the Subscribers dashboard):

Returning Subscriber: If he or she was previously a subscriber, churned for at least 1 day, and has now opened a new subscription. Either subscription could be/ have been in trial or on any plan and the subscriber is still considered returning.

Since upgrading the subscription would not expire and churn the existing plan but rather just switch the existing plan with the new plan, then the subscriber would NOT be considered a returning subscriber, and would not contribute to the Subscriber Growth calculation.