I have used a heat gun to heat up the stuff that holds the glass lens to the plastic housing (concentrate the heat on the glass lens, not the plastic housing). Once heated sufficiently, I pried the lens off the housing. From what I recall (been several years ago, the last time I did it), I didn't have to use any additional sealant when I put it back together. Just heat it up again, to get it sticky, then push and hold the lens onto the plastic housing, until it cools off.
The reason I was doing it back then, I was replacing broken lenses on lights that had good housings (I was using good lenses from lights that had broken housings).