If you want to redeem those IOUs through bitcoin-central, you can just go through bitcoin-central's withdraw process. The IOUs will be exchanged during the payment process.

Otherwise, you should first drop the credit you extend to the issuer whose IOUs you want to get rid of to signal that you want to get rid of those IOUs. Otherwise, you may just get them back immediately. This will also hint to other people that you prefer bitcoin-central IOUs to bitstamp IOUs. Other people who need to go in the other direction will then preferentially use you, giving both of you the IOUs you prefer (this isn't fully implemented on the network yet but should be in the next week or so). That way, they pay some of the fees.

To actually force the IOUs to change without making a payment to someone else, you'd need to either take an offer with the same currency on both sides or create a circular payment path through someone else that pushes IOUs back to you. (Or create a payment transaction that does some combination of these.) I don't believe either of these are supported in the client yet.