You can do so any time. Your old subscription is terminated at the moment of confirmation of the payment for the higher tier. The remaining unused money for the old subscription transfer to the new one and the payment term is postponed according to the sum recalculated according to the new subscription.

Example:

Albert has a monthly subscription to the Silver tier of Locus Map. This subscription costs him 1 € per month, and it renews on the first of the month.

On April 15, Albert chose to upgrade to the annual version of the Gold subscription, which costs 24 € per year.

Albert’s Silver subscription ends immediately. Since he paid for a full month (April 1-30) but upgraded halfway through the subscription period, half of a month’s subscription (0.5 €) is applied to his new subscription. However, since that new subscription costs 24 € per year, the 0.5 € credit balance pays for only one week (April 16-23), so on April 24, he is charged 24 € for a new subscription and another 24 € on April 24 of each year following.