Clarity on the Xbox All Access Purchase Plan & Scarlett

It is not clear that when the option is available to upgrade to the Scarlett if any of what you have paid rolls over or if it's like a rental until you get the Scarlett. Another posing question is do you have to purchase a new Xbox now in order to upgrade
or can you just buy it when it releases next year? And why do you have to finance a new Xbox in order to upgrade instead of just buying a new Xbox right now and then upgrade with trade-in?

:)