Can you give examples when plan is upgraded/downgraded and when quantity is upgraded/downgraded?
When the proration is disabled and the plan gets upgraded or downgraded in the middle of the term, the whole charges would be collected from the customer. For eg- If a plan initially costs $30 and after 20 days, the subscription price is upgraded to $50, then the customer would be charged $50 for the remaining 10 days of the term.
On next billing the customer would be charged $50 for the whole term. If proration is enabled, the customers would be charged only for the remaining days and would not be charged the whole amount. So, he would be charged around $10 in this case.
When a quantity is downgraded and the proration is disabled, no refundable credits would be created and the next renewal amount would be decreased whereas with proration, refundable credits get created for the reduced quantity.
Hope this helps. Let me know if you have any questions.