I transferred my music library from one Mac (running iTunes 11 on Mountain Lion) to another Mac (running iTunes 10 on Mountain Lion). This has worked fine, with one major exception.
When I plug in my iPhone, which was previously syncing with my old Mac, to my new Mac, I get the warning that this iPhone is synced with another Mac. I'm given the option of either erasing it and syncing it, or doing nothing. I would be happy to let it erase and sync since all of the content that is on the iPhone is on the new Mac. However, the vast majority of the 64GB of content on my device is music, and I really don't want to have to go through all of the playlists and artists and albums and configure which ones get synced. (My iTunes library is currently 30k+ tracks, so you can see that it's impossible to fit it onto my iPhone.)
Other than manually rebuilding this information, is there any other way to do this? All of the content that's on the phone is on the new Mac, so I'm not trying to transfer content off of the iPhone. I just don't want to have to go through and manually configure what syncs to the iPhone and what doesn't. I have the same issue with my iPad 2 syncing with the new Mac: same error message, all of the content is on both the iPad and the new Mac, I don't want to have to rebuild the list of what syncs and what doesn't.
I still have the old Mac and its old iTunes library. I would try to reimport the iTunes library, but I've added a bunch of content to the new iTunes library before I tried to sync with my iPhone. Moving to iTunes 11 on the new Mac is not an option. I'd rather take the time to rebuild the sync lists rather than do that.