Upgrade subscription from Free to Paid (is it considered as an upgrade or not)

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Two pricing plans for the Shopify app.
One plan is free

and another one cost 15$ per month.
According to the documentation, I can't create a subscription with a price set to zero(0).
So when a user decides to change pricing from a free to paid, should I create a subscription with the starting the day (today) or would it be upgraded in this case? and should I consider doing this?

Let's say:
Shop install my app on 1st November but on November 15th he will upgrade to a paid plan(assuming that I did not create a subscription for a free plan)
Should I start this new paid subscription from 15 or from the 1st of November?

I want to understand - do I need to reset my stats(backend calculations e.g. messages per month, or emails per month, period) for paid subscription on the last day of November(30th) or the 15th of December?
I hope you understand my question.