Can I get clarification on paying for a Premium Class seat on a Saver fare ticket? This will be our first time flying AS, and I would like to use my companion cert.

So if I purchase Saver fare tickets, do I need to upgrade to Premium Class while I'm doing the booking? Or can I upgrade later by managing my booking? I would like to use my AS credit card for the purchase of the tickets, and then later use my Amex credits for the Premium Class seats. I just wanted to make sure this is possible?

Thanks!