As you upgrade, the system credits you with the unused remaining amount and then uses that credit towards the new payment.

For example: 

If you have an Essential annual plan ($2280/year), which you've subscribed to 100 days ago and decided to upgrade to an annual Pro plan ($5520/year), the system would credit you with 365 - 100 = 265 days or $2280/365 * 265 = $1655.34 and charge the difference of $5520 - $1655.34 = $3864.65 making a subscription anchor on this day.

Did this answer your question?