These are 2 suggestions we found while implementing make:auth for the new system (symfony/maker-bundle#736):

Impact on a custom login form authenticator (as generated by the new maker):

• Automatically add PasswordUpgradeBadge if there is a user password with valid password credentials.

   // ...
   return new Passport(
       new UserBadge($userIdentifier),
       new PasswordCredentials($password),
       [
  -        new PasswordUpgradeBadge($password),
           new CsrfTokenBadge('authenticate', $csrf),
       ]
   )

  Note that this does not automatically migrate all passwords: it still relies on PasswordUpgraderInterface to be implemented on the user loader/provider.
• Add default implementation of AbstractFormLoginAuthenticator::support()

  - public function supports(Request $request): ?bool
  -  {
  -      return self::LOGIN_ROUTE === $request->attributes->get('_route')
  -          && $request->isMethod('POST');
  - }

cc @weaverryan @jrushlow