I just discovered this new change on a flat rate job and I'm with you - I'm not happy at all.
You don't just go around messing wtih people's money w/o telling them first. In the 6 years I've worked via oDesk, I have never seen this ill planned procedure. There has always been some kind of notice, at least 30 days or more.
Now I have to hope my client will agree to pay the $20 I was shorted. That may not seem like much to some people, but to many of us $20 makes a difference in eating.
I am not happy.