None of the below information is hi! I can't seem to get past the link. It just brings me to plans but I have been a premium family subscriber for a while. I tried to switch it to premium and that didn't work as well.
It does have a little screen that says if you are in Quebec you are not eligible, but I'm in Ontario.
I've sent a message to @spotifycares on twitter, which seems to be the primary support contact (eyeroll) to see if there is a way for Family subs to take advantage of this. Will report back with results
The only thing I can think is that for $9.99 per month with Premium you get one account, while with the Family plan you get up to 5 accounts for $5 more. If each were on their own account it would be $50 per month.
I can see the logic behind it, but I still think it's dumb, they would be better off having not offered the promo to existing accounts.