App charges
To know about when the charges are made please read the link below or watch the video.
The Shopify help page does have details for this, here.
Switching your plan during a cycle
When you switch your subscribed plan in the middle of a cycle, an additional charge based on the plan monthly fee difference and the days remaining will be added. For example, here's what the monthly bill will be when changed from Basic ($50/month) to Pro ($180/month) in the middle (16th day) of your cycle.
Basic plan monthly fee: $50
Extra charge（Difference of the plans divided by the remaining days）: (($180-$50)*15)/30 = $65
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Total monthly fee: $115
The same calculation will apply for downgrades as well.