Before I start I haven't read everyone else's answers (just a random few) cus I'm lazy so apologise if I repeat anything!).
I think it is possible to effectively change who you are, but I think it's very difficult. I went to uni with a view to changing, but I reverted back to my old ways (then again I'm lazy…).
I think with enough effort and willingness to change though, it would definitely possible, although whether anyone would really want to change and effectively live a lie I don't know.