How to sync iPhone to a different iTunes library without losing content on phone

I've been using iTunes on my Macbook Air mostly for podcasts, which I periodically clean up to conserve disk space. I had a separate iTunes library on an iMac that holds my complete music collection. My iPhone is synced to the library on the Macbook Air. I just recently purchased an external hard drive and moved my whole iTunes library to it, combined with the few albums that I imported from the MacAir library, where the whole shebang is now available via Airport Extreme to both the iMac and Macbook Air, i.e., both computers can now access the same complete iTunes library, and the podcast-only library has been deleted from the Air. But now my phone won't sync to the library on the external drive. I'm not getting the "Sync and Erase" message that I've read about, but I can't drag content onto my phone, either. How do I get the phone to recognize this new library, and will I have to erase all of the music on the phone to do it?