With your concern, there are 2 cases as the following:
I. You have not reached the current plan limit yet (the translated word limit)
In this case, when you upgrade from LOWER to HIGHER, the charge is prorated based on the difference in price and the number of days remaining in the billing cycle according to Shopify Policy.
You begin a 30-day billing cycle on a STARTER plan, which is $17.9, and then upgrade to an ESSENTIAL plan, which is $69.9 plan on day 10 of the billing cycle.
=> You'd be charged ($69.9 - $17.9) * ((30-10)/30) = $34.7
Simply, you upgrade from STARTER to ESSENTIAL on the same day, you'll be charged for the difference $52 only.
II. You have reached the current plan limit (the translated word limit)
When you upgrade to HIGHER plan:
As the same rule with the first case, your charge is also prorated based on the difference in price and the number of days remaining in the billing cycle according to Shopify Policy.
When you downgrade to LOWER plan:
Especially, in this case, you will NOT be able to downgrade to LOWER plan until your current billing cycle end.
There will be a popup to notice you about this important information as you can take a look on this article.
On the day which your current billing cycle ends, you could downgrade your plan and approve charge, of course, you will be charged as a new billing cycle with the pricing plan that you downgraded.
NOTE: one billing cycle charge is counted as 30 days. (From the day of your approve charge processing)
I hope this will be clear your concern. Let me know if there's anything else I can help you with.
If you need some more helps, please feel free to contact us via live chat or email us at firstname.lastname@example.org