Good point. Some sights are easier to remove and replace than others.If the gun came with night sights, they can usually be replaced for a fairly reasonable price. I just today put a set of Meprolights on a new to me Glock 19 for $47.
Unfortunately, a lot of the trade in or older guns dont have the easily replaced dovetailed sights, so youre limited in what you can do, short of spending more money than the gun may be worth.