SAMPLE AUDIO (free to hear): Click on this LINK.
But simple (cheap:) studio programs like Garageband do not have the built-in capability to do this. I figured out an
1. Set your signature info (tempo, 4/4 or 3/4 e.g.) for Part 1.
2. Create all the tracks for the first part of the song.
In the track name include original info. Example:
3. "Bounce out" (like sending to iTunes) the Part 1 song.
4. In the Project info box, change the signature info to what you need for Part 2. You will see the tracks for Part 1 SHIFT - that's ok; they'll go back if you later revert to the original settings.
5. Now -- important, NOT UNTIL you do step #4 --
IMPORT the bounced out song of Part 1 into a new track.
6. Starting where Part 1 ends (which you visually see from the Step #5 imported track):
Create all the tracks for the 2nd part of the song.
---- 2 measures in 3/4 which lead to the switch.
And the transition is easily completed.
Other examples can be found at my main Music website