So what are developers of existing paid Chrome packaged apps suppose to do for existing users? "Starting in late 2016, newly-published Chrome apps will only be available to users on Chrome OS." As someone who has been building a Chrome app this sudden change with a lack of notice has made me very nervous.
According to the link, in short, it says to notify your users (assuming you've been keeping track of them via a mailing list) and then posting an uninstall button. As far as I can tell, there will be no way to programmatically transition users and their IAP outside of your chromeapp. Meaning, we will have to figure out a way to get users to register their purchases to a database.
It will be very disappointing if Google doesn't help make this migration easy.