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.