I don't remember when or where, but I remember hearing someone say something about how it was interesting how Paul sort of became the lead singer of The Beatles (as opposed to John being the lead singer early on). For me, a quick mental scan of the albums confirmed it, but I wanted to be sure, and to see it. So, I used this cool Wikipedia page that lists Title, Year, Album, Author(s), Lead Vocal(s), Chart Position UK, Chart Position US, and Notes for every one of their songs..
..to make an Excel document. In the Excel document I listed their UK releases chronologically by date recorded, not date released ("Abbey Road" was recorded after "Let It Be" but released before, etc) with a column for John and a column for Paul. Whoever was listed as lead singer between the two, I gave a "1", then totaled at the end of each album. (There were some songs I had to make an executive decision on. Songs such as "A Hard Days Night" say, "Lennon and McCartney," so I gave them both a "1", but songs like "A Day In The Life" say, "Lennon, with McCartney," so I only gave Lennon a "1")
I wanted to know for sure and I wanted to see it, but I wanted to know for conversation's sake where the transition took place, if it did at all. I was also interested in a definite transition, which I found.
Please Please me had Lennon eight, McCartney seven, which was close, but John out lead-sang Paul by at least three songs on the next three albums, and stayed ahead until Revolver, when something interesting happened: John and Paul tied for lead singer credit, with five each, for the first time in the band's UK LP history! Wanna know what else? On the rest of their albums, unless you count "Yellow Submarine", (which I don't, I'll explain later), Paul led in lead singer credits. Although they are both lead singers, it is clear, to me, that John was the lead singer until "Revolver" when they tied, and Paul was the lead singer after "Revolver".
I don't count "Yellow Submarine" because, of its 13 tracks, seven are orchestral arrangements composed by George Martin, their producer. The other six are actual songs, but "Yellow Submarine", the song, appears first on "Revolver", and "All You Need Is Love" appears first on "Magical Mystery Tour". That leaves you with four new songs. By today's standards, that's an EP. One is a Paul-led song (All Together Now), one is a John-led song (Hey Bulldog), and the other two are George-led songs (Only a Northern Song, It's All Too Much). So they tie. They tie on one album after "Revolver", but for the sake of who was the lead singer at any given point, I don't can't "Yellow Submarine". Actually, I hardly count "Yellow Submarine" as an album in any scenario.
So, with my data entered, I made a chart. I still had the data listed for "Yellow Submarine", and "White Album", which is a bit skewed number-wise, because it's a double album, but here's what the chart looked like
A bit hard to comprehend, given the "Yellow Submarine" and "White Album" abnormalities, but you can see the lines cross at Revolver, then blue never goes below red again. I wasn't satisfied, so I made a new chart, sans "Yellow Submarine" and "White Album"
It's really obvious here that red is ahead of blue until "Revolver", where they tie, or cross, and blue is ahead of red after "Revolver".
So in summary, for this specific investigation, you can think of John Lennon as the lead singer of The Beatles until "Revolver", and you can think of "Paul McCartney" as the lead singer of The Beatles after "Revolver"..you can think of them both as lead singers on "Revolver".
1. This in no way is me saying Paul becomes better than John or that Paul is better than John or anything. I like Paul more, but I LOVE John Lennon, and more importantly, I love love love The Beatles, as a band. Paul and John are equals in my opinion, as far as everything Beatles.
2. This information makes me wonder, "Why the shift?" Did Paul just get really prolific and demanding all of a sudden? Did John run out of ideas? Well, I think you can watch Paul's ego develop if you dig into lots of video on YouTube or watch the Let It Be movie, but I think the explanation is something else. I don't think John ran out of ideas, because he had an extremely prolific decade or so after The Beatles, featuring many timeless songs. I mean, he made writing a song like "Imagine" look easy, and it probably was for him. What I PERSONALLY think happened, is that John lost interest in the band. Think about "Come Together". It's pretty much the only Beatles song that sounded like that, and it was on their last album; maybe evidence he was interested in going a different direction. "Plastic Ono Band", his first album after The Beatles, was totally not Beatlesy in my opinion. By the end, I could imagine John considering The Beatles as kid stuff or been there/done that.