This is a valid question, but it is too broad, as it stands, to be useful.
If what is meant by 'AA' is the spiritual principles of the Twelve Steps, these are obviously ever unchanging: the principles of admission of defeat, inventory, confession, restitution, reliance on God, and service are unassailable.
If what is meant is the 'clear-cut directions' in the Book, then we have a different question entirely. I have sponsored people whose minds do not work very well. I have sponsored people whose minds work very well indeed. The basic instructions stay the same, but how the ideas get conveyed, the depth of understanding attained in Step Four, and even some aspects of the mechanics will vary substantially from person to person, without the principles being compromised. This IS one-size-fits-all ... but the programme shapes round any individual to meet them where they are at the time they give themselves to the process. As a friend of mine called Bob says, 'this programme is Bob-shaped'.
The 'clear-cut directions' described in the Book were obviously not precisely what even our founders used. Page 263 gives a wonderful example of how Dr Bob took people through. It is quite clear that this does NOT conform to the instructions in the Book. There are many who would say that this is not AA, if someone were to do this today.
If what is meant is the content and format of any particular meeting, then we have a real problem. Which meeting is 'the' model? A particular meeting in November 1937 in New York? The meeting on the same night in Akron? These would have been radically different! Which one is real AA?
The conservative approach is essentially nonsense because there is no original gold standard to adhere to. The arguing over the writing of the Book is splendid testament to this.
Any attempt to reduce the whole of AA down to a mechanical set of over-simplified instructions which must be followed by the little tin soldiers of AA marching to the beat of a martinet sponsor is inconsistent even with the early days of AA.
AA has always been a cloud of bees buzzing round a set of principles, guided by an unfathomable inner instinct and resource which we presently identify with the power greater than ourselves that restores us to sanity. The closer we adhere to the principles, the more successful we are. The further away we drift from the principles, the less successful we are. Experience is the teacher.
God's creation is a manifestation of constantly shifting, constantly changing variety, with everything obeying basic rules of physics. AA is much the same.
A glance at any spiritual tradition will reveal an emphasis on a dynamic tension between eternal principles and the shifting circumstances of the present.
If it ain't moving, it's dead.