YES, you can switch any time. The cost of the upgrade is pro-rated, so the sooner you upgrade, the more will be applied towards it.

Upgrading is simple. If you decide that you'd like to upgrade from ie. monthly to annual, simply go here, and then to the far right of your Membership, click the Change Plan link.

That will send you to the standard purchase page but the prices you'll see are prorated based on what you've paid already, and how long you have had that membership for. So, the sooner you upgrade, the more you'll be credited.

IMPORTANT! Please note that if you have a subscription paying via the app, on Apple Store or Google Play then you cannot do a pro rata upgrade sorry. This page is only for website subscriptions where you can see the 'change plan' button on your subscriptions page.