It's not guaranted that score will increase. But as per my experience. It's good option to give a shot especially for speaking section. There is high chance you could get convert 8 to 9 in speaking.
I strongly recommend to do that.
Hey
Definitely you should go for re-evaluation. I got increment of 1 mark in speaking.
Everyone should try Re-Evaluation thing. It really work for at least 1 mark increment.