What was the case against not letting them alter it at all once set?  As in, if they want to start issuing new debts with a new fee they can create a new address.  My issue is that the cost of transferring the debt would be a key ingredient in how I value it.  One of the better things I'm seeing in Ripple is that it would give debt issuers less control over the transference of their debt, and less ability to suddenly make arbitrary rules or fees about how that happens.  Fees would be set up front.  They can charge a fee, perhaps they can even decrease it, but I don't see why you would allow them increase it.  Particularly as you (being Ripple) ultimately want people to keep their IOUs in the system rather than cash them out.