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?
As I understand, lowering your trust does not trigger any money movement. Bitstamp will no longer be able to send you IOUs, and nobody else will be able to give you their IOUs either. The next time you send anybody BTC, the Bitstamp BTC will be sent, if they trust Bitstamp.