Well call me cheap and wreckless, but as someone who has rebuilt lots of industrial equipment over the years, I am of the opinion that if it is as new as you say, and has not been seriously stressed or damaged during operation and/or removal, I do tend to re-use gaskets and even O-rings when they are only a couple weeks old. With gaskets, sometimes I may grease them or spray them with Silicon spray to sort of rejuvenate them a little, so they will compress and seal properly when reinstalled.
I am doing the same as you. I am pulling my upper to replace a shift cable here next week. I just put the upper back a week ago to grease the U-joints. If nothing tears, and the O-ring is fine, I will also re-use them. It hasn't even been in the water yet, so that is also why.
