The one doctrine that absolutely must stay is the New Light doctrine. This is the alibi foundation for changes to all the rest.
Other religions change a teaching -- take Catholics on eating meat on Friday, remember that one? Ooohh -- what a contradiction. We'd tell the householder that one day Catholics were in Purgatory for eating meat on Friday, next day they weren't.
Same with Mormons. One day multiple marriages are okay, next day they're not. Contradiction. Of course, we always neglected to acknowledge to these householders that Mormons had abandoned their teaching some 50 years before Jehovah's Witnesses began disfellowshipping (1947) for that same practice.
It's obvious that JWs believe that they have a world patent on the New Light doctrine -- that they're the only ones entitled to alibi their way out of changes to their teachings.
As for me, I love it when they knock on my door nowadays. I always get a chance to divulge the fact that I studied with them many years ago. Not a lie -- I simply don't expose the fact that I then became a MS, an elder, developed a bothered conscience that resulted in disfellowshipping.
Their inevitable question -- "Why did you stop studying?" -- is the perfect lead to my introduction to The Great Watchtower Contradiction, an 11 minute read designed for new ones but which poses questions on topics that the most seasoned JW would cringe at.
Contradiction contains a table handout that you can snip for purse or billfold. I always keep my billfold handy.
Len