I'm not sure why the price affects the changeability other than as stated in the rule nonrefundable amounts remain nonrefundable. See also this thread where someone reported they successfully changed after the start of the trip:
http://www.flyertalk.com/forum/onewo...midtrip-2.html
And in this post a person reported that they changed a dcir26 to a dcir29 after the trip commenced:
http://www.flyertalk.com/forum/18164770-post115.html