switching from Steam to client version.

This error can sometimes occur when the new email address you are trying to use is already linked to another Path of Exile account. If the email address you request is already linked to a Path of Exile account, you will either need to choose another email address for your Steam account, change the email address associated with your secondary account, or if you prefer we can delete that account for you.

If you'd like that account deleted, feel free to contact us at support@grindinggear.com and we'll help you out. :)