Why can't I remove this trim board?
The z flashing probably has a good pinch on the top of that 1x12. Also dried paint likely has bonded to the surface 


Try to get a thin putty knife worked under the lip to separate the flashing from the trim board. 
Be aware that your idea for a new trim board will only work if the new one is same thickness as the old one. 
Try to avoid deforming the z flashing.