If you just want to put new tubes in the tires, don't bother to take the caps off. Unseat the outer bead of the tire, and get it to the outside of the rim. A little plastic tire tool for bicycles works well-Walmart may even sell them. Take the old tube out, and off over the end cap. Before you put the new tube in the tire, put just a little bit of air in it to so it can't get pinched as easily as a completely empty tube.
The old plastic tire tool doesn't have as big a slot as these have in them, but it will give you an idea. http://www.amazon.com/Ventura-Bicycle-T ... B001KEH9MM
These are more like mine. http://www.amazon.com/Park-Tool-TL-4-Le ... d_sbs_sg_5
You put them inside the tire, using the rim as the fulcrum, slide them around a little, and pry the tire over the rim. Continue to slide one around, completely getting the tire bead to the outside of the rim.
When I was in a tire store getting new tires on a horse trailer, I saw one of the mechanics changing a rear tire on a semi-trailer. He never touched the lug nuts-just jacked the trailer up, and zipped the tire off the rim.