"If you can login with your old password you can be sure hackers, using this exploit, don't know it. "
No, all that you know is that any hacker who knows your WiFi password hasn't changed it yet. They could only do that by also gaining the admin password.
That's not really a problem though because without knowing where you live and getting close enough to receive a usable signal they're unable to do anything about it. If someone has your WiFi password their chance of using it is 1 in tens of millions, with each chance requiring a change of physical location.
Remember it's only the WiFi password, not the admin password - there's no way to change the router settings - including the WPA passphrase - without the latter.