This doesn’t upgrade your account to Premium. The upgrade adds your account to an existing Premium For Family account, meaning there are some settings you can’t change.
It probably also means whoever the ‘Family’ is you’re getting added under can see what you’re doing. If you’re fine with that, it was a pretty seamless transaction.
I won’t be upgrading again in a year though. I was under the impression this upgraded my account to Premium and left it at that. At no point did I see that I was going to be under someone else’s account.