Watch the The I-vi online guitar lesson by Ravi from Guitar Lab: 30 Rock & Pop Progressions
The I-vi progression uses two chords that are similar to one another in sound and often in fingering. The vi chord (minor chords are written in lower case roman numerals) is also referred to as the "relative minor" of the I chord (major chords are written with upper case roman numerals). Therefore, the change is subtle, so subtle that sometime the vi is used in place of the I (as you will see in other segments) rather than in conjunction with the I.
Sometimes using the minor 7 chord (m7) in place of the minor makes the I-vi chord change easier to play in certain positions up the neck. When playing the I as a barre chord on the 5th string, you can easily get to the vi minor 7 without moving your hand, just a couple of fingers. Also, by varying the strumming pattern or even using your fingers to pluck the strings instead of strumming with the pick, you will add even more individuality to particular songs.
Examples: Pretty Woman, Locomotion, My Sweet Lord, Matter of Trust, What's Going On