Banks would need to take control of the 51% hashing power and propose to move decimal places from 8 to 9 (or so). But to understand what I've just said requires that you change your thinking from deflationary bitcoins to inflationary satoshies.
It is irrelevant to interest rates in deflation problem. If you borrow 1 BTC you have to give back 1 BTC no matter how many decimal places were added to protocol in meantime.