The question was whether "major structural changes" can, as a matter of plausibility and possibility, arise in microevolutionary (creationist definition) steps. IOW if a "major" structural change can arise in stepwise fashion without any of the individual steps being "major".
Clearly and inarguably the difference between reptilian jaw bones and mammalian ear ossicles is both "major" and "structural". Equally clearly a stepwise transition between the two, without any step being "major" is at least possible because we actually have examples of the stages. Of course mainstream scientists think those examples actually are steps in a real transitional series, but even if you reject that the mere fact of being able to arrange even a hypothetical transition, if with real fossils, shows at least the possibility of achieving "major" change in "non-major" steps.