I think you can UPGRADE from the MONTHLY PAYMENT MONTHLY to the ANNUAL MONTHLY PAYMENT, but not the other way around. You'll need to talk to customer support about it.
Adobe provides support through the Chat function on the contact page, through a phone call, or through Twitter. There is no email support.
CHAT: Click the following to contact Adobe customer support staff for help:
https://helpx.adobe.com/contact/support.html
You must be signed in with your Adobe ID.
You must allow cookies in your web browser for this to work.
Disable any ad blocker
Disable any pop-up blocker for the site
Disable any script blockers for the site
This is the link to the US website. If you are not in the US, the site should redirect to the page appropriate to the region associated with your ID.
Click the chat icon at the bottom right of the page to open a chat session.

If that doesn't work, try this link, which opens a chat window directly:
https://helpx.adobe.com/contact.html?rghtup=autoOpen
PHONE: You can also search this page for a regionally appropriate phone number for customer support.
https://helpx.adobe.com/contact/phone.html
You can also try Twitter, @ AdobeCare. (take out the space)
Meantime, I'll move this to the Account, Payment, & Plan forum, which is the appropriate one for questions of this sort.
* The Using the Community forum is for help in using the Adobe Support Community forums, not for help with specific programs, installation issues, or account issues such as subscription questions or billing problems, or generalized questions about Creative Cloud services.
* Questions about business plans should be posted in Enterprise and Teams
* Product questions should be posted in the associated product community.
* Installation questions should be posted in the Download & Install community.
* Account issues, including subscription questions or billing problems, should be posted in the Account, Payment, & Plan community.
* Questions about the Creative Cloud desktop app or general questions about apps in the Creative Cloud should be posted to the Creative Cloud Services community.