Turns out it can be pretty hard indeed, when you discover that the original fixture was on top of the paneling and wallboard, but the junction box was completely inside the wallboard, with a nearly inch long gap spanned by extra long mounting screws instead of the fixture fitting flush on the box. The new fixture doesn't mount on the standard junction box mounts; it mounts to a crosspiece (provided) which mounts to the standard mounts, with screws which aren't long enough to reach across the extra gap, and of course are a different diameter from the extra-long screws which held the original fixture up. I suppose I could have made a trip out to a hardware store and bought extra-long screws that would fit, but in practice that would mean the job wouldn't be done today, and I really didn't want to put the old fixture back. I managed to find a way to use the extra-long screws to secure the crosspiece outside of the wallboard and use the provided screws to attach the fixture.
Hopefully the house won't burn down.
In other news, I finally made concrete progress on the MEFES newsletter that I was supposed to have in the can a couple of weeks ago at least. I'm not quite sure why I've found it so hard to apply myself to this, but I've just been putting it off and putting it off. But the last minute I can work on it before FKO will be Tuesday, and if it's not done before I leave, we won't have a newsletter before the meeting next month.