TLDR: You should be able to change the element of your exotic with one exotic shard at the gunsmith, but it would be a menu similar to xurs upgrade menu. I believe that with xur or the gunsmith you should be able to change your weapons element to something else with the cost of an exotic shard. This would be similar to the xur upgrade process. In example the gunsmith would have a menu of exotics that would refresh each week. You could then choose the gun you want to change to. But the element is rolled as well. So if you wanted a void gjallahorn and the arc version was on the menu, sorry. I believe this would be a good thing to do because it adds more variety to the guns. Please bump this so that bungie will read.