There is a _COOKIE_KEY_ in PrestaShop that is used to encrypt the password before saving them to the database. The _COOKIE_KEY_ for each PrestaShop installation. The _COOKIE_KEY_ is unique for each and every PrestaShop installation.
You can find the _COOKIE_KEY_ in following paths:
/app/config/parameters.php (in PrestaShop 1.7) /config/settings.inc.php (in PrestaShop 1.6)
You should set the passwords for your users on your new PrestaShop 1.7 store again using the updated _COOKIE_KEY_
You can take a look at Tools::encrypt() function to find out how it is encrypted.
Note: This _COOKIE_KEY_ is called 'secret' in PrestaShop 1.7