Syllabus change with no warning nor discussion with students worsened grade
I would let the professor know that you had been deliberately putting more effort into larger assignments with the understanding that the coursework was worth 10%, and point out the pattern in your marks as evidence of this.  You could ask that the 10% of project marks be distributed over the other project marks rather than the coursework marks.  Your professor may be willing to do that as an exception for students that complain.

But I wouldn't go in angry and suggest that a contract has been violated.  There should be a mutual goal between the students and the professor to do the best they can at both education and marking / ranking.  Probably a lot of students were happy to get more credit for work they'd already done.  Spending a lot of time on something that presumably doesn't affect your GPA that much may not be that good of a strategy, or that professional of behaviour.

(How much better were your assignments than your homework?  And then this difference is only on 10% of your mark for one course?)

Of course, if you are near a borderline and this really will make a big difference to you, go talk to your professor honestly about this problem and how the decision disadvantaged you. Do this during their office hours.