Downgrading is NOT closing/opening an "account".
The "account" stays the same and the "age of the account" is not impacted. The only thing that is closed and opened are the credit cards. Big difference between "account" and "credit card".
Example:
1) Downgrade Dividend to Double Cash. Account intact. Account age intact. Dividend card gone, Double Cash appears. New card number if necessary.
2) Close Dividend, apply for new Double Cash. Account terminated. New account created. Dividend card gone, Double Cash appears. New card number.