Before
booting up Logic, try moving your AU's out of the Components folder to
a temp one. Bring up both songs. The one with the plugins will have
the AU plugs with a line through them (ie they won't load).
you also get an message alert box for each instance of each plug. I just opened an older song last night and got half a dozen of those warning windows. but it doesn't take too long to hit return each time.