What you said is what is in my mind, but since the account is no longer useful for him or her, I think the best option is to delete all the post made and leave it empty since is not useful, beside if it happen to change the email address the account will still belong to the user, that's my opinion
You can probably just abandon it altogether, I mean are there any identifier that you don't want people to be seeing in that account that deleting it is the only way? Maybe you can do what you are saying but that is a pretty tiresome thing to do compared to just abandoning it.