Once you have bought the EDU version it is no different from a regular version. In other words - you have to upgrade to a regular version as well, there is no option to upgrade to another EDU version.
However, it is still much cheaper to upgrade from C9 to C14 than buying a new version.
Don’t forget to check all the infos regarding the new licensing system which will come in May 2025.
Afterwards you can use all versions from C11 downwards with your dongle as long as your dongle does not get smashed and all versions from C14 to C12 without your dongle.