Do I just rearrange the elements or is there some kind of technique?
I am looking to make 300x250 into 728x90.
For example, for this tower type banners I've noticed the headline is on top than image.
With having much more room could you not rework the idea to fit the new size maybe split test some new ideas (on the same banner) while you getting there?
Unless your banner is a total winner i would say i could be a dire waste of the extra room if you don't use it for juicy creativity.