when you use the loadVar command it should load whatever you specify into the area you’ve specified regardless of what’s already there. for example if reg1.txt is loaded into a text box in your movie, when you tell the movie (by clicking your button and activating the script) to load the new .txt file it should automatically put the new one in it’s place (in other words, unload the old and load the new)
although i’m affraid i don’t use the loadVar command often, but you might have some luck doing a search in the top right of this site for tutorials on text boxes. there should be quite a few