While nothing was set in stone, The Beatles definitely had a lineup. John Lennon played rhythm guitar; George Harrison played lead guitar; Paul McCartney played bass; and Ringo Starr played drums. When that changed, there was usually a specific reason.
For example, when Paul took over at the drums on “Back in the U.S.S.R.,” he did so out of necessity. After getting fed up with the band during the White Album sessions, Ringo had left town. Paul was simply filling in to keep the train rolling.
When Paul took the guitar solo instead of George on Revolver’s “Taxman,” that happened mostly for efficiency’s sake. In those days, the band — and producer George Martin — didn’t plan to spend as much time on George’s songs as they did on songs written by John or Paul.
Source: cheatsheet.com