I see you've opened a support ticket for this. We'll respond there with the details for your specific case on that ticket but I wanted to provide an explanation for how we handle upgrades/downgrades.
When you change from one subscription to another your billing account is given credits for any time remaining on the previous subscription during the upgrade or downgrade. Those credits will be used first before generating new charges. If there are surplus credits those will be used up the next time you're billed.