What **** I'm going to loose exactly, when I sync my iPhone with "different" computer?

Sorry for using crude language, but no freaking patience, when google-fu didn't brought any results; what files I am exactly going to loose, when I'm syncing my old iPhone full of my personal infomation (pics, videos, contacts, messages, emails, ton of app relative cache data, so on) with my computer?

I have synchronized my iPhone before, however, it was the same computer, different account (which doesn't exist any longer). iTunes is giving hard time accepting this little detail, and while wanting to sync some new songs on my iPhone, nope, no can do without emptying and then syncing my iPhone. Which is naturally out of question because earlier mentioned amount of data loss in so many freaking places.

Also, tips around "Sync music to iPhone WITHOUT using iTunes, wuup wuup" are welcome as long as they don't include subscription for service to do that (i.e. Spotify).