Different question. I've granted 0.6 BTC trust to bitstamp and hold 0.3 BTC/bitstamp. I was fooling around and dropped my trust to bitstamp from 0.6 BTC to 0.2. My account showed that I still held 0.3 BTC/bitstamp, exceeding my trust limit. Shouldn't lowering trust levels below existing IOUs owned have triggered some process whereby 0.1 BTC/bitstamp was immediately returned to bitstamp and converted into BTC, thereby bringing my balance in line with trust granted?
"If I tell the system that I don't trust bitstamp to exchange my BTC IOUs for real Bitcoins, should Ripple automatically trust bistamp to exchange my BTC IOUs for real Bitcoins?"
Of course not. If you don't trust them, the last thing you want to do is destroy the IOUs you hold and trust them to actually send you the Bitcoins since that is exactly what you just said you didn't trust them to do.
You can use those IOUs to make Ripple payments and exchange for other IOUs and nothing Bitstamp can do can stop you. You can let other people take those IOUs from you and in return give you IOUs from issuers you do trust. If you don't trust them to redeem them, that's what you want to do with them.
If you do still trust them to redeem the IOUs and are lowering the trust because you don't want to acquire any more IOUs (maybe you're switching gateways) or fear that they'll stop redeeming in the future even though you trust them now, you can follow up by withdrawing the IOUs using the gateway's normal interface.
How would I technically go about exchanging my bitstamp IOUs for -say- bitcoin-central IOUs?