No, a tarantula cannot change sex, if that guy from the expo is reading this please take note.
It is sometimes hard to sex a spider for various reason
1. You simply dont know how to sex a tarantula
2. You sexed a tarantula to young and thought you saw a spermatacea (how ever you spell it)
3. Some spiders are hard to sex because they keep destroying there molt.
I know Poecilotheria Formosa is hard to sex because it tends to destroy its molt pretty often and
is difficult spider to sex ventrally. I have heard of this species mating male with male. Its
more weird to see them both tap for each other