Your right. It will not do multiple at once!

Here's something you can try, find your .gpx file and right click it and then click on "open with" and then find notepad and select it. You can uncheck the box that makes this the default opening program for now. Once open you can find your wpts by doing a "find and replace" from the menu. Just type in the first few letters or numbers of your points and then type in what you want to change it to then just keep clicking next till done. You can make it to what you want and leave the numbers at end to help manage them.
I am able to change mine to the first three letters of each lake I was on and then the number. Just save the file as "*.gpx" and not as a .txt file. You can save it as a new file if you like and then see how it looks before you load it up.
Hope you find something.