Yes, you may upgrade at any time.
Step 1:
Sign in to your Disney Account on D23.com.
Step 2:
Select “My Profile” from the Membership dropdown to access your Account information.
Step 3:
Select “Modify/Cancel” from the left menu.
Step 4:
Select the “Upgrade Today” button to upgrade. Follow the prompts to confirm. Congratulations! You have upgraded your D23 Membership.
Notes: Upgrading your Membership mid-cycle does not extend your existing renewal date.