What's the cheapest way to cube an item from Rare?

Now the question becomes, what scroll, if any will you get back if using a Guardian Scroll in conjunction with an expired Unique Potential Scroll?

I guess this extends to non-expired, time-limited UP scrolls too. Will you get the same one back? Will you get an extension by seven days? Will they become regular UP Scrolls, tradable with no expiration date? Will it not work? Will it eat your 2K NX and not work?

So many questions, some obviously more plausible than others, of course.

It probably just prevents the scroll from being consumed.
I'll probably check next time I scroll with Guardians to see if the scroll disappears from my inventory for a second.