Light fixture holes don’t align with junction box

Before you start, make sure the power to the fixture is off (not just at the switch) and, if possible, confirm with a non-contact tester (a few bucks at a hardware store).

D are the screws that need to be put through B, which is the bracket that will be attached to the ceiling box to hold up the fixture.

To see which holes, align the bracket with the holes in the fixture base. Thread screws D through the back of the bracket so the threaded ends stick out front. Which side of the bracket is the back depends on the ceiling box and whether you need the offset on the bracket sticking down or up. (It often doesn't matter.)

C are the screws that hold the bracket B to the ceiling box. Once the bracket is installed, you can connect the wires on the fixture to the wires in the ceiling box (including the ground wire to the green screw on the bracket).

Place the fixture against the ceiling with the threaded ends of D sticking through the holes. Attach with the nuts E.

The threaded tube on A is screwed into the center of the fixture, the nut is then tightened down the tube to the fixture. The remaining parts are removed. After bulbs are installed, the globe is put on, followed by the soft washer, the hard washer and the next nut. Don't overtighten or you can crack the globe. Then the decorative cap is put on and then the ball finial.

A sawzall should make quick work of this. If you don't have one and know no one that does, they can be rented. There are also handles that clamp onto hacksaw blades so you sort of have a small, manual sawzall. It will take some work, but you will be able to cut enough that the remainder can be twisted off or chiseled aside.

If done carefully and methodically, a dremel tool can do much of the work without starting a fire. Try to direct the cut so that the sparks are ejected mostly downwards. Stop often and check the area for signs of burning. Have a spray bottle handy to quench any embers, as well as a fire extinguisher just to be safe. When finished cutting, spritz the area with water and check the area frequently for the next hour for any signs of embers developing.

Finally, depending on the finish, cutting and patching the ceiling is not that big a deal, then you can ensure the new box is properly supported instead of relying on the little tabs of the old work box and ceiling material for support.