[y46ukgrc2n6]

I attempted to do exactly the same operation on Google Music and it was even worse. It wouldn't show the option to upgrade to a family plan anywhere. Turns out that signing up for Google Music All Access also gets you YouTube Red, and that YouTube Red subscription has to be deleted before you can upgrade All Access to the family plan (which also gives you a new YouTube Red subscription). It took a conversation with a google support rep to figure this out, and I think someone who is not a programmer would have a harder time understanding this failure mode.