In short: Yes.
You can upgade or downgrade your subscription between the Standard and Exclusive Memberships at any time, as many times as you like. To upgrade, you will be taken through the checkout process for the higher membership fee; it therefore won’t make sense to upgrade more than once per month.
To upgrade or downgrade, login to your account and navigate to your account, click on the ‘Subscriptions’ tab and ‘View’ the subscription you would like to switch from. Under ‘Subscription totals’ you will be able to click to ‘Upgrade or Downgrade’. Select the subscription you would like to upgrade to and proceed to checkout.