First off, you will never lose your (purchased) music now, due to iCloud. It'll always be there, even though in case of catastrophic HD failure, you may have to redownload them all. However, that is not your case now: you just want to know if deauthorising one comp of 5, will prevent you from listening to your old songs.
No, it won't. Go ahead.
I had used up 4 of my 5 authorisations, but decided to to start at zero with 10.5, without any loss to apps/music/playlists, etc.
P.S.: ALWAYS backup your important stuff unto an external HD. You will never regret doing so. Trust me.