I can see it. Remember Churchill's quote, "Those who are conservative at age 20 have no heart, and those who are liberal at age 40 have no brain." It's certainly possible that grew a little wiser as he got older.
Lennon was famously iconoclastic and liked to tweak authority. Being in the music industry, he certainly knew his share of smug, know-it-all lefties. It's not far-fetched to think that he may have grown sick of those types.
In addition, there were some leftists (not enough) who re-examined their beliefs in the aftermath of the Vietnam War as they became aware of the atrocities committed by the victorious Communists.