Password expiration polic

Hello I have an environment in which we have 20k users. 19k users are synced from local AD. 1k user in cloud only (printers, services etc.). The issue is that password are not expiring. From documentation i understand that for those synced users is pretty simple - configure msoldirsyncsettings, CloudPasswordPolicyForPasswordSyncedUsersEnabled - after those actions i can force password expiration user by user. But what concerns me the most is actually the first step - setting up the expiration policy in admin.microsoft.com. What will happen with those cloud only accounts after i set this setting? Will they stop working until i change password on each of them? Do you know how to minimize the impact in such environment?