Transgender/Transsexual

I was having a private conversation with someone who identifies as transsexual and not as transgender recently, when the thought occurred to me: is that possible? I understand the need to identify as transsexual (the book I’m currently reading, Max Wolf Valerio’s The Testosterone Files, has made that argument quite well) but I really wonder if it’s possible.
I mean can your gender actually stay the same when you change sex? I don’t think so. I think probably a lot of butch lesbians come pretty close when they transition, because their gender is essentially already male. (Not all of them, mind you, but some.)
But gender is so variable. Unless you’re talking about a two-gender system, where the only choices are masculine and feminine, then perhaps a very gender non-conforming person who is trans but pre transition might be gendered the same way before as after transtion. But I can’t imagine that being true for any MTF I’ve met, to be honest; the taboos against males expressing femininity are so drastic, that it’d take one seriously (brave, irreverent) feminine male-bodied person to truly inhabit a feminine gender while living as male.
So my feeling is that being transsexual and transitioning usually implies also being transgender.