If it was me I would errect two sections of staging (scaffolding) to safely get up there to work on this safely. Home Depot Tool Rentals have various size scaffoldoing for rent as so do other rental yards; two sections will get you up to 12′ and three walk boards will give you one comfortable staging platform at 6′ and two for the top working deck. Be sure to get the casters (wheels) as not only will it provide you with an additional 6" of height, but they will prevent any damage to your floor.

Also, go into the electrical department of Lowes or HD and purchase what’s called a "Safety Brace" electrical box. There is both new construction and remodel versions that securely fasten between the ceiling joists and are specifically designed for heavy lighting fixtures and ceiling fan installations. They cost under $20 and are very easy to install.

Mike Lonergan
TradesProfessional, Inc.
http://tradesprofessional.us or .com

Obviously you need to get up to the ceiling. What you use will be determined by the area available. You might be able to rent a folding ladder at one of the home supply stores. 20 feet is quite a reach. If you can find a reasonable solution to the height problem, there shouldn’t be too much trouble changing out the light.

Getting there is the problem. Once there, the fix is relatively simple.

I have the exact same issue in my home, but it is 18ft high. You need to get up there for changing lights, removing cobwebs, and repainting the ceiling someday. What I did is built a freestanding ladder, it’s like an A-frame step ladder but I made it 12ft high. It is a little risky but works well for ceiling access. In your case I would make it 14 or 15ft tall. I used as thin of wood as I could trust to hold my weight. This keeps it fairly lightweight for getting it in an out of the house and garage.