Currently, we do not have a way to transfer student accounts from one Teacher to another. We also do not have a way to convert a student account into a regular Scratch account. We are still in the process of thinking through what this flow might look like and apologize for any inconvenience this causes you. In the meantime, we do have a few suggestions that would allow your students to keep their work.
One way for students to be able to keep their account and projects would be to leave the class "open." Students only lose access to their accounts and projects when a class is closed.
If you do not wish to keep a class open, we would encourage you to have the students create a new (regular) Scratch account. Once they have created this account, there are a couple ways they can keep their projects. The first way would be to download their projects from the student account and then re-upload them into their newly created account. Alternatively – and perhaps more easily – they can make use of Scratch's remix feature and remix their projects from their student account to the new account. Remixes will be saved even if the original project is deleted.