Let's make our application backwards compatible. We're lucky because we started the application with fragments (or at least should have), so the transition to supporting tablets and phones will be fairly painless.
At this point we already have the support library integrated. Swap your Fragments for the support version (from the v4 library). You'll need to update your manifest to support devices back to API level 7 (aka. Froyo).
Your biggest challenge with backwards support will be the ActionBar. It may also be tricky to migrate Loader's.
Make sure to test on older devices either through Genymotion or with test devices.