Why Lennon and McCartney lyrics changed?
I am like a chameleon, influenced by whatever is going on. If Elvis can do it, I can do it. If the Everly Brothers can do it, me and Paul can. Same with Dylan. John Lennon
I could feel myself climbing a spiral walkway as I was talking to Dylan. I felt like I was figuring it all out, the meaning of life.” Paul McCartney
In their early lyrics, Lennon and McCartney broadly stuck to the Tin Pan Alley format: essentially, "I love you/please love me too."
There were some exceptions. Paul's When I'm Sixty Four, for example, shows the influence of music hall and takes a very unusual perspective for a teenage songwriter.
Generally, though, the words circled back to the trials and tribulations of romantic love: I Want to Hold Your Hand, From Me to You, etc.