want to change to cc student plan, but the option is not supported

You may need support (this is not support, it is a public forum)

Be sure to remain signed in with your Adobe ID before accessing the link below
-make sure that your browser does NOT block ads, scripts, or pop-ups
-you must also allow 'cookies' in your web browser for this to work
https://helpx.adobe.com/contact/support.html for chat or https://helpx.adobe.com/contact/phone.html
-click the chat icon at the bottom right of the page to open a chat session
-the chat icon looks like '3 dots inside a circle' at the lower right
-type AGENT into the chat window to connect directly to a person rather than the AI
Do be aware that Adobe's staffing is impacted by the virus, so wait times are long
.
Also: beware of answering anyone who sends you a private message
-click for more information about scammers https://tinyurl.com/10791730

And more information

Start at http://www.adobe.com/education.html
-Cloud Education Plan https://creative.adobe.com/plans?plan=edu
-http://www.adobe.com/creativecloud/buy/students.html
-FAQ https://helpx.adobe.com/x-productkb/policy-pricing/education-faq.html
-One Year Intro Price https://community.adobe.com/t5/Get-Started/Student-plan/td-p/10209357
-Redemption Code https://creative.adobe.com/educard
-https://helpx.adobe.com/x-productkb/global/graduation-education-discount-expiry.html