Apple’s Mastered for iTunes “format” is supposed to provide music as the artist and sound engineer intended. In general, if you have purchased music from the iTunes Store in an older format, you can’t get the newer tracks without buying them again.
However, the iTunes Store is allowing you to do this for tracks by The Beatles.
Go to this page to find out more.
This is an interesting change. However, beware: Mastered for iTunes tracks may not sound as good as you think. Keep your older tracks just in case.
Update: the direct link above doesn’t seem to be working. Try going to The Beatles’ page on the iTunes Store then clicking the link you see here: