Upgrade from lower to higher subscription before current subscription expired.

From SimplyBook.me

If you need to upgrade your subscription just navigate to Plans&Prices//Subscriptions page and click on Upgrade* under the desired plan. Make payment and you will be able to use the options you need.

If you select lower subscription you will get Buy* option and your next subscription will become active when your current subscription expires. It is done to protect our users, because if the subscription is changed right away, the current subscription bookings will simply disappear. That is why, the next subscription gets activated only after the current one expires.

*It may be necessary to cancel recurring payment first in Plans & Prices -> Payments history -> Recurring payment profiles