- Make sure that you have a the version of all apps that are compatible in your iTunes library.
- Then backup your library to an external HD or similar
- If you every need a compatible version just delete an incompatible version from your iTunes app library and then copy a compatible version to your Apps library
- To copy compatible versions from your iPod to your iTunes library first delete incompatible version and then:
In iTunes 11 use Control+B to show the Menu bar then go to iTunes>File>Devices>Transfer Purchases