Depends on the horse itself and how it is trained. I turned my barrel racer into a jumper, I found no obvious problems coming from the fact that he was trained western first. He was able to go back and forth with ease.
I also had another horse that was trained western pleasure, that I was working on trying to get to go english. He grew frustrated because he was set in his ways to be western pleasure, and when I started working on him to carry himself more up hill and on the bit. He grew frustrated fast and I had issues, but that was his personality. He didn't like change too much.
I agree with the above post. Depends on the horse. I changed my horse (19 at the time) from western to English. As far as I know he was used as a western horse all his life. He changed with no problems.
MOST of the horses I bought for my tiny riding academy were Western trained, and they all worked English, too. If you intend to show, then you'll have to experiment regarding what English discipline would best suit him.