Paul McCartney and Ringo Starr made an emotional and powerful statement with the release of their final song, "Now and Then" ... using new tech to clean up and boost a demo recording Lennon made back in 1978.

In a short film accompanying the song, the icons explain that John recorded his vocals ... and 14 years after his murder, his widow Yoko Ono handed over the tapes to Paul.

They tried to release the song in the nineties, but with poor muffled audio, the track "languished in a cupboard" for years and was eventually shelved -- until now.

Artificial intelligence has restored the song, with fans now able to hear Lennon clearly sing, "I know it’s true. It’s all because of you. And if I make it through, it’s all because of you." The voice of the late George Harrison is also featured on the track.

Unlike many other biz projects made in memory of those who have passed, Lennon's family is thrilled with the final product. His son Sean says, "My dad would’ve loved that, because he was never shy to experiment with recording technology. He would’ve loved that."