Changing personal account to free school account

you would need to have both accounts for the time needed to share the undergrad account with the grad account and load the files to the grad account.  ie, make sure all the needed files are on your personal computer and then add them to your grad account's sync folder.  after all the files are in your grad account, you could cancel the undergrad account.

to cancel that undergrad account, http://helpx.adobe.com/x-productkb/policy-pricing/return-cancel-or-change-order.html 

if that undergrad account is still under its one year obligation, you can expect a cancellation fee but might be able to get it waved because you have another (the grad) account.  it would be up to adobe support to determine that, so you should cancel via them.  using a browser that allows popups and cookies, contact adobe support by clicking here, https://helpx.adobe.com/contact.html?rghtup=autoOpen
in the chat field (lower right), type AGENT
be patient, it can take quite a while to reach a human

p.s. if you're contacted by anyone (via email or private message), it's much more likely to be a scammer than an adobe representative. ie, double check for an employee badge if contacted in the forums and look for an adobe.com domain in the email address if you click reply to an email. then check again and be very suspicious. any other method of contacting (or offering to contact you) is almost certainly a scam, https://community.adobe.com/t5/using-the-community-discussions/staying-safe-in-the-adobe-support-community/td-p/12919684/redirect_from_archived_page/true.

p.p.s you can also use phone (but chat is usually quicker), https://helpx.adobe.com/contact/phone.html