If, as Isaiah says, God's thoughts are as far removed from ours as the heavens from the earth (Isa. 55:8-9), then Mormons believe we should expect our understanding to change radically as we grow deeper into our divine nature. While the basic teaching of the LDS Church have not significantly changed, there have been some drastic changes in some LDS practices (polygamy, etc.) due to continuing revelation. Living on the this side of those changes, we may forget how profoundly many Saints had to alter their thinking about what they had understood until then to be revealed, unchangeable doctrines.
By the same token, Mormons cannot rule out the possibility that teachings they now regard as fundamental may someday be revised, even abandoned, as God leads the Saints to new understanding. Reliance on the scriptures or teachings of living prophets must be accompanied by the recognition that understanding of truth is subject to change, even on questions for which God may appear to have given decisive answers.
So, are Mormons flip-floppers? I don’t think so. As a child grows and their minds sharpen, their once held beliefs evolve. So it is with a church and people that believe in continuing revelation. It is not an exercise in flip-flopping, but one of understanding that increases.