Extending premium account

[Guest]

Suppose you buy 10 months of premium account. Then you get a display at the left of the screen like

Thanks for your interest in the Premium Program!
Premium valid until <expiration date>

The system will still allow you to purchase a premium account. Will newly purchased months of premium account extend your expiration date, rather than canceling the months you have already purchased?

[Guest]

They will extend your premium account.

In your example:
If you invest another 20 Coins your premium account would be valid not only for ten but for eleven months.