as far as my studies on physiology have gone metabolism has no memory. if you do a diet and have a break long enough for the body to adjust to a new one it wont remember the before. it just adapts to what is in front of it.
the reason the atkins works based on the theory of our ancient diet is not a memory of life as hunter gatherers its simply that we havn't evolved fast enough to keep up with the diet.
the reason i believe people say its difficult years after the first attempt is 2 fold 1 lifestyel 2 as you age your metabolism slows.
when they were younger maybe had no kids etc or young kids (lots of running around) now they are older lower metabolism not got the same active lifestyle so the weight comes off slower and hence more disappointing and disheartening.
its not possible nor practical to compare your body years after the first attempt due to wear and tear.
To infinity and beyond!