More than a few on here probably have. I myself have changed mine when I swapped the header, and then again when I swapped back. The duration that you have them on does not matter. The issue is that they compress and take shape and do not go back when removed. Your exhaust is rigid and the flanges aren't 100% parallel to the exhaust ports. The gaskets fill the voids when compressed. If you don't put new ones in, there is a high probability of a leak which of coarse can cause other issues i.e. popping, backfire, heat, etc. It's much easier to put new ones in when you are changing the headers than to have to take them back off again to replace them.