From their Pre-Order page..
"Regarding the evolution of the price for a pre-order product:

If the YEN price of a pre-order product has been down, you can ask us to update the price of your order with the new price. (or a refund of the difference for preorders already paid).
This is valid only before the official release of the product."

I think that is pretty good. If you preorder in yen, your preorder price will not go up but if it goes down and you wish to pay it while it is lower they will let you do it. If it goes back up, you missed you chance but you still have the original preorder price as a safety net.

Mind you, this is what i read and i have had no experience in trying to lower their price.