Perhaps it was some other type of document then -- there is no option for a Voluntary Reroute on a FIM (see #9 below):
May be the case for LH, but not for AC. I've had a FIM issued at least 3 times for this precise reason, and after having paid the $75 change fee because I was travelling on an e-ticket and had didnt have enough segments to change to a new routing.
Note BA also has one with an "other" box after listing delayed, misconnect, cancelled and oversold.
http://ext.pimg.tw/ttwu/1208874693.jpg