BUG #16239: It is not possible to upgrade the password from md5 to SCRAM

The following bug has been logged on the website:

Bug reference:      16239
Logged by:          Valeh Agayev
Email address:      valeh.agayev@gmail.com
PostgreSQL version: 12.1
Operating system:   CentOS Linux release 7.7.1908 (Core)
Description:

It is not possible to upgrade the password from md5 to SCRAM with \password
command, but with alter command is possible.

postgres=# SHOW password_encryption;
 password_encryption 
---------------------
 scram-sha-256
(1 row)

postgres=# SELECT rolname,rolpassword ~'^SCRAM-SHA-256\$' AS has_upgraded
FROM pg_authid WHERE rolcanlogin;
 rolname  | has_upgraded 
   ---------- +--------------
 u1           | f
 postgres | f

postgres=# \password u1;
Enter new password: 
Enter it again: 
postgres=# SELECT rolname,rolpassword ~'^SCRAM-SHA-256\$' AS has_upgraded
FROM pg_authid WHERE rolcanlogin;
 rolname  | has_upgraded 
 postgres | f
 u1           | f
(5 rows)


postgres=# alter user u1 encrypted PASSWORD '123';
ALTER ROLE

postgres=# SELECT rolname,rolpassword ~'^SCRAM-SHA-256\$' AS has_upgraded
FROM pg_authid WHERE rolcanlogin;
 rolname  | has_upgraded 
----------+--------------
 postgres | f
 u1       | t