How to use previous iTunes icon?
There are many ways to do this. Here are a few...

IMO The best way is to replace the actual AppName.icns file with the one from the older version, in this case it's: /Applications/iTunes.app/Contents/Resources/iTunes.icns

Select the application bundle and control-click (right-click) > Show Package Contents, then navigate to the AppName.icns file shown in the pathname above.  Note:  After replacing the AppName.icns file one may need to run killall Finder in a Terminal to force the updated AppName.icns file to show properly everywhere.  If it's not showing properly in the Dock afterwards run killall Dock in a Terminal.

Another way is to paste an image into the App's Info Sheet after selecting its icon at the top left of the Info Sheet.  Select the application bundle then control-click (right-click) > Get Info or  ⌘ -I.

Or use a third-party app that can assign an new image/icon to the application bundle.