If you change password policy from medium to strong: what happens with those users who are not complying with a strong password today? Will they be kicked out or will it just work until they are forced to change password accrording to "Password Expiration in Days" ?

I'm fairly certain it's the latter, their password will be valid through the existing period, and they'll be forced to change it into compliance when they do a password reset.

Okey, that is what I think too. But I would like to have it confirmed. I'll open a case with Netsuite.