hi, don't worry about it, there are no problems.
you can switch and switch back as many times as you want.
amazon lets you use basically any card with any store, they handle the currency conversion, so it doesn't matter if you use you uk/french/etc card on the usa/japanese amazon store.
here are a few images i made to help someone else who was worried too, to show all you need to do.
http://postimg.org/gallery/bk7cyacg/8839fa1c/