Upgrade and Downgrade Charges Calculations

Example 1

Last billing date is on May 1. Next billing date is on June 1. Subscription fee of the old plan is $10, charged before billing period. Subscription fee of the new plan is $20, charged before billing period. Upgrade is done on May 11.
Upgrade order sum is calculated as follows: $6.67 = $20 * 20/30 - $10 * 20/30
($20*20/30) is the amount billable under the new plan from the date of upgrade through the next billing date.
($10*20/30) is the amount already billed under the old plan from the date of upgrade through the next billing date.
Billing order sum on June 1 will be $20.

Last billing date is on May 1. Next billing date is on June 1. Subscription fee of the old plan is $10, charged before billing period. Subscription fee of the new plan is $20, charged after billing period. Upgrade is done on May 11.
Upgrade order has zero value.
Billing order on 1 June will have value of $6.67 = $20 * 20/30 - $10 * 20/30
($20*20/30) is the amount billable under the new plan from the date of upgrade through the next billing date.
($10*20/30) is the amount already billed under the old plan from the date of upgrade through the next billing date.
Billing order sum on July 1 will be $20.

Last billing date is on May 1. Next billing date is on June 1. Subscription fee of the old plan is $10, charged after billing period. Subscription fee of the new plan is $20, charged before billing period. Upgrade is done on 11 May.
Upgrade order has value of $16.67 = $20 * 20/30 + $10 * 10/30
($20*20/30) is amount billable under new plan from the date of upgrade through the next billing date.
($10*10/30) is amount billable under old plan from the last billing date through the date of upgrade.
Billing order sum on 1 June will be $20.

Last billing date is on May 1. Next billing date is on June 1. Subscription fee of the old plan is $10, charged after billing period. Subscription fee of the new plan is $20, charged after billing period. Upgrade is done on May 11.
Upgrade order has zero value.
Billing order sum on June 1 will be $16.67 = $20 * 20/30 + $10 * 10/30.
($20*20/30) is the amount billable under the new plan from the date of upgrade through the next billing date.
($10*10/30) is the amount billable under the old plan from the last billing date through the date of upgrade.
Billing order sum on July 1 will be $20.

Last billing date is on May 1. Next billing date is on June 1. Subscription fee of the old plan is $20, charged before billing period. Subscription fee of the new plan is $10, charged before billing period. Upgrade is done on May 11.
Upgrade order sum is calculated as follows: -$6.67 = $10 * 20/30 - $20 * 20/30
($10*20/30) is the amount billable under the new plan from the date of upgrade through the next billing date.
($20*20/30) is the amount already billed under the old plan from the date of upgrade through the next billing date.
Billing order sum on June 1 will be $10.

Last billing date is on May 1. Next billing date is on June 1. Subscription fee of the old plan is $20, charged before the billing period. Subscription fee of the new plan is $10, charged after the billing period. Upgrade is done on May 11.
Upgrade order has zero value.
Billing order sum on June 1 is calculated as follows: -$6.67 = $10 * 20/30 - $20 * 20/30
($10*20/30) is the amount billable under new plan from the date of upgrade through the next billing date.
($20*20/30) is the amount already billed under old plan from the date of upgrade through the next billing date.
Billing order sum on July 1 will be $10.

Last billing date is on May 1. Next billing date is on June 1. Subscription fee of the old plan is $20, charged after the billing period. Subscription fee of the new plan is $10, charged before the billing period. Upgrade is done on May 10.
Upgrade order sum is calculated as follows: $13.33 = $10 * 20/30 + $20 * 10/30
($10*20/30) is the amount billable under the new plan from the date of upgrade through the next billing date.
($20*10/30) is the amount billable under the old plan from the last billing date through the date of upgrade.
Billing order sum on June 1 will be $10.

Last billing date is on May 1. Next billing date is on June 1. Subscription fee of the old plan is $20, charged after the billing period. Subscription fee of the new plan is $10, charged after the billing period. Upgrade is done on May 11.
Upgrade order has zero value.
Billing order sum on June 1 is calculated as follows: $13.33 = $10 * 20/30 + $20 * 10/30
($10*20/30) is the amount billable under the new plan from the date of upgrade through the next billing date.
($20*10/30) is the amount billable under the old plan from the last billing date through the date of upgrade.
Next billing order sum on July 1 will be $10.

Last billing date is on May 1. Next billing date is on June 1. Subscription fee of the old plan is $10, charged monthly before the billing period. Subscription fee of the new plan is $50, charged quarterly before the billing period. Upgrade is done on May 11.
Upgrade order sum is calculated as follows: $4.44 = $50 * 20/90 - $10 * 20/30
($50*20/90) is the amount billable under the new plan from the date of upgrade through the next billing date.
($10*20/30) is the amount already billed under the old plan from the date of upgrade through the next billing date.
Billing order sum on June 1 will be $50.
Billing order sum on September 1 will be $50.

Last billing date is on May 1. Next billing date is on June 1. Expiration date is on January 1 next year. Subscription fee of the old plan is $10 per month, charged for the subscription period. Subscription fee of the new plan is $20, charged monthly before the billing period. Upgrade is done on May 11.
Upgrade order sum is calculated as follows: -$63.33 = $20 * 20/30 - $10 * (20/30 + 7)
($20*20/30) is the amount billable under the new plan from the date of upgrade through the next billing date.
($10*(20/30+7)) is the amount already billed under the old plan from the date of upgrade through the expiry date.
Billing order sum on June 1 will be $20.

Last billing date is on May 1. Next billing date is on June 1. Expiry date is on January 1 next year. Subscription fee of the old plan is $10, charged after the billing period. Subscription fee of the new plan is $20/month, charged for the subscription period. Upgrade is done on May 11.
Upgrade order sum is calculated as follows: $146.67 = $20 * (20/30+7) - $10 * 20/30
($20*(20/30+7) is the amount billable under the new plan from the date of upgrade through the expiry date.
($10*20/30) is the amount billable under the old plan from the date of upgrade through the next billing date.
There will be no billing orders after upgrade.