People should be eating high fiber foods everyday, in significant quantities. You body does adapt. If even small quantities of these foods causes issues, then you can try increasing slowly and consistently, over say a month or two. Or you can try changing your diet quickly if you have time to deal with any potential side effects short term.
So, just make sure to cook the grains and legumes properly, don’t worry about how cooking degrades nutrients, and eat lots consistently. If one methods of cooking digests or tastes better for you, stick to that.
It’s like people who ask why exercise hurts after being sedentary for a long time. Just exercise more, and it won’t hurt.
The mechanism you are describing about fat cells refilling vs making new cells, does not make sense. They have done studies, and the number of fat cells remain constant during adulthood, unless you undergo liposuction or other procedures. I think what you are describing is a set point weight, where your body tries (via hormones I think) to maintain its preferred weight. So its more a metabolism effect rather than physical space issue about storing fat.