As for changing the email address, I just reassign a new cookie in MyAccount.aspx (below in bold). You suggest to delete the cookie it, while the only way I found to do this is SignOut method (is SignOut called a method?) which is not what we want to do, obviously.
Un.WebModules.Accounts.Business.User currentUser =
if (currentUser.EmailAddress != EmailAddress.Text)
FormsAuthentication.SetAuthCookie( EmailAddress.Text, true );
What do you propose for the logout when you are on the MyAccount page? I tried to search deja.com for Validation/SignOut issues but to no avail. I instead created a LogOut.aspx page, and converted the SignOut button to a HyperLink control.
If I find something I will let you know, cheers from the DR Congo,