Disney+ Promotion (24 month subscription) : Premium instead of Standard?

I signed up for Telus Mobility, and was offered 24 months of free Disney+ subscription. Appently it is only the Disney+ Standard tier ($11.99/month), not Disney+Premium (14.99/month). 

I would like the Premium suscription, and I am willing to pay the $3/month difference.   If I sign up for Premium, will I still receive the 11.99 as a credit on my monthly bill?

I was told to use this website to sign up: 

https://www.telus.com/en/shop/subscriptions/disney

Alternatively, should I sign up for Standard and try to upgrade somehow after the fact?