I prefer leaving my drawings up to interpretation because everyone has something a bit different they like to see.

With that said, I -- the artist -- am not a big fan of mental changes because of your reasoning exactly: why bother with a TF when the same thing is accomplished by just replacing the character? Leaving out the mental changes lets the character react and show off their personality, not the personality of the new character.

So, in the vast majority of my drawings, the subject is aware of the change and is not "replaced" by the end character. They don't always enjoy it, though, so this guy got off easy!