IMO, it should be (and remain) Lennon & McCartney. Here's why I believe this:
"THEN" -- John and Paul had this agreement when they were young and, for whatever reason, John won out and had his name first. I think it made sense a couple of ways - Lennon started The Beatles, and it's alphabetical.
"NOW" -- You don't monkey around with tradition and history, especially when it comes to The Beatles. I think Paul's getting a little concerned as he approaches old age that he won't be remembered for what he wrote, but what's done is done, and it's Lennon and McCartney, which is classic and the way it must be (like Simon & Garfunkel, Abbott & Costello, Laurel & Hardy, Gilbert & Sullivan, etc.... etc....)
If they had thought more about it way back when, it would have made sense to put the name of the Beatle who wrote most of the tune FIRST, or bill the person individually if there was only one writer. But Lennon & McCartney made this pact, and it should stick.