According to at least one trans person I know, you become a transsexual when you "go under the knife." She was talking about SRS/GRS--which, until recently, served as most people's line of demarcation between transsexuals and everyone else.
A friend of mine is scheduled to undergo that procedure next year. Yesterday, she sent me a text message announcing that she was in a hotel room, recuperating from facial surgery.
I congratulated and welcomed her.