Forward Existing Email Account and Don't Retain

It looks to me that the email forwarder when you forward and existing email account, it says it will "copy" the email to the forward address. I don't want to delete the email account and lose all the pre-existing mail, but I want to forward all future email for a while. I don't want to have duplicate email on both accounts. It's okay that no new email is retained on the original account while it is being forwarded.