Stupid me broke the nut off my Presta valve on my front tire, with the nut in the closed position. Obviously, I need to change the tube now but I can't get air in and can't get air out as far as I can tell. Ironically, I've never had a flat tire before so this will be my first opportunity to test my tire/tube changing abilities. From what I've read, however, it's really difficult, if not impossible, to get a tire off unless the tube is deflated. Any suggestions?

I may be able to avoid the whole mess if my new custom wheelset gets here before the weekend. I can put my new wheels on since I have new tires and tubes for them and let the tube on the current wheel just deflate over time before attempting to change it. Here's hoping.