The collagen improvement and refacing of the epidermal texture appears to be long term from Fraxel laser, which is clinically similar to other procedures that were investigated with biopsies more than ten years after a procedure, such as dermabrasion.
If you are happy with the results of your Fraxel treatment and the way it improved your acne scars, the chances are that your satisfaction with the procedure will persist. Of course, nothing is permanent. But you could (and should) have the procedure repeated if necessary.
As you mentioned, there is loss of volume as time goes by, therefore the results gained in new collagen deposition will also decrease. Whether the scars reappear as original, probably not because fractional resurfacing does change the tissue, essentially vaporizing portions of the scars and replacing them with new collagen.