Professor Changed exam questions weights after test was done

Ok , so my problem is simple , we had a test that’s out of 25 , 5 questions ( 5 points each ) + 1 bonus question ( 3 bonus points to a max of 25 ) .
My problem is this , I’m 99% of the time confident of my answers and have a 4.0 gpa , and it’s just a simple differential equations class, the first 5 problems are marked out of 5 on the cover page and at the header of the problem page. I breezed through the test to the point I didn’t even need to solve the bonus question since it was a very very long problem , longest in the test actually , that would take 30 minutes atleast ( 2 hours test ) and it’s only giving 3 bonus points I don’t need.

While taking the test I specifically asked the instructor about this problems weight since it didn’t make any sense to me the hardest and longest question would be marked out of 3 while the others are marked 5. He said it’s just the bonus question there’s only 3 points .

I didn’t waste my time with it , handed my paper , 3 days later grades come out and he shows us the papers … he marked down the first 5 questions weight to 4 points , and marked up the last one to 8. Instantly losing me 8 points , so I end up with 20/25 , effectively losing the A from this course as the max I can get after this is 94/100 total ( there’s 2 marks for attendance , you only get 2 if you miss 0 sessions , 1 any other number of missed class , 0 if you don’t attend 50% of the sessions ) . And there’s no curves in our university.

So is the professor allowed to do this ? What can I do at this point ? ( just adding in that appeals rarely result in anything )