It has now been proven by forensic science that Paul was replaced in 1966. A scientific team in Italy conducted a biometrical study of Paul pre & post 1966. The scientists saw big discrepancies in the facial features. The article says that to account for the differences, Paul would have had to have undergone a series of extensive & painful surgeries, but the requisite scars were not visible. Neither was his singing career impacted. Other proof of Paul's replacement is different eye color (brown/hazel to green) & height changes (he grew 2+ inches after 1966).

Chiedi chi era quel «Beatle»
August 2009 WIRED (Italian)
[link to Do_not_use_this.com]

( long url: [link to www.wired.it]

It seems they are saying that too much of "Paul's" head and face changed for that person to be Paul. There is too much difference between the skull, palate, mouth, nose, jaw, chin, & eyes. Paul would have had to undergone a series of major surgeries. He would have had to wear wires and braces which could not have been hidden simply by growing a mustache.