I always see people online and in real life ask "I've been riding english my whole life and I want to ride western but I think it will be hard", "My horse has been trained in western so she can't do English or dressage", Or people will go look at a horse to buy and ask what disciplines they can do and the owner says to them "Western, this horse doesn't have a English bone in his body".
The truth is that pretty much the only difference in these disciplines is what tack they use and what he/she wears. Will a horse excel more in one discipline than the other?, Yes. But any horse and any rider can do any discipline. Dressage is the basis of all riding, if your horse is under saddle and riding it's doing some type of dressage. The major difference in disciplines is showing and how you are scored.