garageband changing chords

The loops in the Loop Browser come in families. You can just cut and trim the loop you selected. And instead of looping it, use a different loop from the same family for the following measure.

Also, to create a chord progression, you can select a single loop, open the Track Editor, select the "Region" tab, then transpose the the loop by the number of halftone steps you want.

For example - the second loop has been transposed by 5 halftone steps:

If you are using the green software instrument loops and not real instrument loops, the track editor will show you individual notes and you can change the pitch of the individual notes to modify the chord. But GarageBand 10 does not come with man software instrument loops like the previous versions of GarageBand.

What kind of built-in Chords are you using in GarageBand? Loops? Are you asking about software instrument tracks or Real Instrument tracks? And which version of GarageBand are you using on your Mac?

For real Instrument Loops you can use the transpose slider, for software instrument loops you can additionally change single notes in the Track Editor.