You can upgrade your annual subscription at any time with no extra money payments. All remaining days of your current plan will be automatically converted into equivalent value days on the new tier. You don't lose anything, just use the remaining amount to switch to a better version. Thus, the remaining period of your subscription will be reduced and your next payment date will be switched. Be aware of it.
Remember, it's not a free trial. Once upgraded, there will be no option to revert this action. However, you'll be able to set a downgraded plan for the next billing period.
Here is an example how the upgrade looks like. To check how it works in your case, click the Upgrade button on the plan you'd like to switch to. Don't worry, nothing will happen without your confirmation.