While these quick bass loop tips are for Logic Studio, the basic concepts are the same in most software. The Acidized and Apple Loops format of Bass On Demand contain pitch information. So to change chords in a song you'll want to change the chords in the Global Tracks timeline in Logic. The screen shot shows the chords going from G to D to C, etc. What you first want to do is set up your chord changes as if you were writing a lead sheet or chord chart. You then drag and drop the bass loop closest to that note onto the track in each measure. In each BPM/Style Song Set, there are a variety of core patterns, alternates and "fills" to choose from. There are low versions of each note and an octave higher for each note as well. Note that in Logic, for audio loops, you do not change the project key in the timeline but in the chord timeline.
For other programs the concept is the same. If your program reads Acid or Apple Loop pitch information (most do), you change the key or chord in the timeline of your project. Some programs also allow you to open up a specific loop in the timeline and change the pitch as well via internal pitch shifting plug ins. In each case you want to select bass loop closest to pitch you are stretching to to get the best quality. Because these are real audio loops it sounds like a live bass player, not a MIDI machine. You get slides, fret noises, etc. Very authentic.
Here are our Bass On Demand demos made with a variety of DOD loops.