cdmckay, , .Identity.Name ,
IIdentity newIdentity = new GenericIdentity(properlyCasedUser.UserName);
if (User is RolePrincipal)
User = new RolePrincipal(((RolePrincipal)User).ProviderName, newIdentity, ((RolePrincipal)User).ToEncryptedTicket());
else
User = new GenericPrincipal(newIdentity, null);