iTunes track Information changes won't stay permanent!

Having spent well over a decade building an accurate iTunes Library, I'm now frustrated that many of my changes, particularly with respect to release dates of music tracks and correction of artwork that was not right when it was first downloaded, are reverting back to whatever they were when I downloaded them from either the internet or from CDs. Years of work is being slowly undone every time I hit "play." What changed? Is there any way to make my changes permanent? This only started happening with the last couple of system updates. It's a terrible waste of time to do hours and hours of historical research only to have it undone every time I want to play some music! Accurate release dates are important to me.