To change the modpack your FTB World is running on, start up the modpack you want to change to then go to the FTB Worlds screen.

Click on your FTB World, then click on the Configure button.

On the configuration screen, you will see a Worlds & Backups button. Click on this to go to the world management screen.

Now click on Create New World this will create a new world for the modpack you are currently running.

You will now see that your new world has been created and has the name of the modpack below the world number. To switch to this world, click on the Switch button, after its marked as Selected, you should be able to exit the configuration screen and play on your world.

You can now swap between your worlds as you want to play on different modpacks.