The epithelium is the outermost layer of cells on the cornea. These are the fastest reproducing cells in the human body. The underlying layers, bowmans, stroma, decements, and endothelium, do not regenerate. Lasik applys laser energy in the stroma, so those cells do not return.
What changes is the internal biomechanical strength of the cornea and the epithelium. With the internal stresses changed by surgery, the cornea will change shape during the healing process. This is a primary cause of regression. The epithelium assumes the changes are unwanted and attempts to "correct" the surgery. From myopic patients under about 6.00 diopters, the biomechanical and epithelial changes are not enough to effect real change in refractive error. Unfortunately for hyperopic correction, both the biomechanical and epithelial changes dramatically effect the refractive error and is why regression after hyperopic Lasik is so common.