Sometimes when a song moves to a new key on the guitar, that change introduces a completely different chord progression. Here, you start with a chord progression in A, followed by a new chord progression in a different key, C. The last chord in the example, E, leads back to A, the tonic of the first chord progression.
“Oh, Pretty Woman” by Roy Orbison uses a key change similar to the one here. In it, you hear a verse with a chord progression based in A major and a bridge with a chord progression based in a different key, C. Both the key and the chord progression change. There’s also a bit of borrowing back and forth before everything finally settles back to the original key of A.
Technically, you could also say that the opening and verse switch tonics within the key of A. The riff (called the Intro and Interlude in some scores) starts on an E chord, giving you the impression that E is the key, before surprising you with a verse centered on A.
You hear something similar in “Summer of ’69” by Bryan Adams, where the bridge presents both a new key and a new chord progression. The main part of the song centers on D and the D major scale, while the bridge centers on F and the F major scale with a new chord progression.
Another example, “Tears in Heaven” by Eric Clapton, mainly centers on A and the A major scale, but the bridge changes to C and uses a somewhat different chord progression.
Coincidentally, all three of these songs modulate up a f3rd at the bridge.
Open up your record collection or favorite media player, because here are more examples for you! “What I Like about You” by the Romantics has a bridge that is completely different from the rest of the song. “Stuck with You” by Huey Lewis and the News changes from C in the verse to D in the chorus with a slightly different progression.
“School’s Out” by Alice Cooper starts in E Dorian but eventually settles into the chorus key of G minor with a completely different progression. “Sweet Child O’ Mine” by Guns N’ Roses is centered in D Mixolydian mode through the first half of the song but switches to E minor and a new progression just past the halfway mark (guitars tuned down one half step to Ef).
When tracking a song with chords that don’t all fit into one key, break the song into individual parts that can each fit into a key. All the song examples here have parts that are almost like mini, separate songs in themselves, put together to form a larger whole.