Yosemite is a Software Update download/install. It's not a "Combo" updater for a later, new version of a current (or even a previous) OS.
I would suggest simply making a compressed or renamed copy of the iTunes version you want to use. Might be best is to copy the whole library and the plist in both ~/Library/Preferences/com.app.iTunes.plist and (possibly) /Library/Preferences/com.apple.iTunes.plist. I'm not sure if there are other files hidden somewhere. The point is, you'll have a copy of the old files, library, and plist, even after you make the update. You can then drag the old files into the appropriate folders and the OS should ask/warn you if you about replacing new stuff with old. You'll likely need you computer password to authenticate that you are doing things against the Will of the Mother Ship!