So, unlike many people I was actually able to get the codes for the youtube/disney offers.
It's then you come across the really frustrating thing:
Neither of those offers (and potentially some of the others, I haven't checked) seem to be available to people with existing accounts (and the youtube one is also unavailable if you've ever paid for google play music, ans I did many years ago.) If you want them you have to completely set up a new account and ditch your old one.
Yes unfortunately you need to unsubscribe and wait until the end of the subscription to enable the boost code. I guess to try and incorporate a 6 month or 12 month free code in an existing subscription will be very problematic. Plus it's on the third party company supplier that the code is attributed to. Just going through this with the Disney offer myself