it will replace all the music and your library with the library you are trying to sync now. it also replaces anything you purchased however which includes your apps.. so you might lose some app data if you do that
but there is this workaround for that: http://www.ilounge.com/index.php/articles/comments/syncing-ios-apps-with-a-diffe rent-computer/
your pictures and videos should stay untouched tho
but just to make sure you can always back everything up to icloud before syncing and then if you lose the photos you can just restore them back from the icloud backup... but it shouldnt delete your photos
Thanks for your reply. I backed up my phone via iTunes with the intention of restoring in case a lot of data/apps were lost. I was not worried about music being erased because it was empty anyway.
I hit the "erase & sync" with fingers crossed. It appears as if nothing was lost. It may have swept the Music app clean before syncing, but again it was empty anyway.
I guess it's all fine for now. Thank you.