I will explain the difference, and I will show how you changed the meaning of the title by changing that word. "Transsexual" describes someone in the process of transitioning from one sex to another. "Transgender" more generally describes someone who does not identify as the gender they were assigned at birth. While it might be argued that all transsexual people are necessarily transgender, you made the substitution in the other direction, going from general to specific. This changes the meaning of the title in a way that is not logically permissible. Consider an article with the title: "An animal bit a man.". It would not be necessarily correct to rewrite this as "A dog bit a man.". However, if the original title had been "A dog bit a man", then the transformation to "An animal bit a man" would still necessarily be true.