Upgrade from lower to higher subscription before current subscription expired.

From simplybook

Your next subscription becomes active when your current subscription expires. It is done to protect our users, because if the subscription is upgraded right away, the current subscription bookings will simply disappear. That is why, the next subscription gets activated only after the current expires.

If you want to upgrade sooner, please contact our support and they will pass your request to techies. In this case, we will convert the rest of your current subscription to the upgraded one and add to your balance.
For example, if you have 2 months of Standard and need to upgrade to Premium, we will add approximately 1 month of Premium to your account instead.