To chunk up a bit and see the forrest for the trees here, think of it this way Phil: no matter WHAT change to your site you are considering (design changes, biller changes, price point changes, update changes, etc) always, always A-B test that change.
Get yourself a simple A-B script (the user Woj here on GFY has a nice simple one, for example) and let the changes roll for a week. If you have enough daily traffic hitting then you can do it for a day or two. Compare, compare, tweak, compare.
