That's retconning, done by Sir Paul to cover up his embarrassment. Listen to it--they're definitely singing "In this ever changing world in which we live in."
I'd always heard it that way myself -- and I'm glad to learn I'm not alone in that regard -- but the surrounding lyrics imply the other reading: "But in this ever-changing world in which we live in makes you give it a try" just plain doesn't make sense.
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