If you want to ride in jeans you can still you are able to find something else, chaps or english pants. I ride in jeans cause Im to lazy to go out and buy chaps. I don't show so it does not matter much.
You might want to think about getting a trainer to help you switch over. Its easy to go english to western but harder to go western to english. The saddle is smaller, you need to post, reining is not the same. Plus your horse will need to be worked with some.
What do you plan on doing in english?