It's not like it hasn't happened at least twice before (see #15 in my FAQ above). Also, it really isn't unusual in projects for that to happen - even actually having to go back and redo work already marked as complete. Thirdly, as
Chaos_Alfa states - progress, in this case, isn't actually going backwards... it just will look like it, based upon the percentage because they are going to try to better define how much each of the remaining tasks contribute to completing the tech update - rather than all tasks having the same percentage, no matter what amount of work is involved.