Paul McCartney and John Lennon were ambidextrous
Left-handed guitarists have never had it easy, especially in the early years of the instrument's U.S. popularity. Reportedly, Jimi Hendrix’s father forced him to play right-handed when he was a youngster out of belief that left-handedness was a sign of the devil. Jimi accommodated his dad when he was around and then flipped the guitar for left-handed playing when he was gone.
Paul McCartney had it somewhat easier. A southpaw, his dad didn't force him to play right handed, but like other lefty guitarists he had to tweak his Zenith acoustic guitar by switching the string order and making homemade fixes to the nut. Even so, over time he managed to learn how to play guitar right-handed given that much of the time he was among right-handed guitarists with no suitable instrument in sight.
“I can play right-handed guitar a bit, just enough for at parties,” he confirmed to Guitar Player in 1990. “Hopefully, by that point everyone is drunk when I pick it up, because otherwise they're going to catch me. But I could do that."
He explained that it would have made little sense to ask if he could re-string someone's guitar. "And at a party, you only want to play it for 15 or 30 minutes or so, and by the time you've goofed up their guitar and you hand it back to them, they've got to string it back again, and it's silly. So I had to learn upside down.”
Source: guitarplayer.com/Elizabeth Swann