Darth frosty is right on! But, it must be worded in the correct way to TEACH.
Ask the question, "Could it be possible that the WTS has always made changes (got "new light") after the prophecies failed because the prophecy failed??
The person answering would be forced to answer yes. If they don't, ask, "You don't think that's possible? Then you're saying that it's impossible?"
Now you have planted some seeds of doubt.....Gymbob