Personally I would stick with what you are used too, unless there is something in Avast One Premium that you particularly want.
I personally don't use any of the Avast One product range and I use Avast Free Antivirus - there are differences in the Avast One range, some say greater range of components. But for me the Avast One Essentials (Free) has less in the way of user configurability and different components.
As far as I'm aware, I don't believe there is a way to purchase under a special offer price, but defer the start period of the license. This is the same as all purchases the licence period is active immediately.
You could of course uninstall Avast Premium and install the Avast One Premium on a trial (not sure if that would be 30 or 60 days) and pay at the end of that trial (or go back to Avast Premium). However, you would lose any difference in the offer price, if that 60 days were greater value than the offer difference.
