So, the wind had finished breaking off the base of three fence posts after some years of rot had weakened them.
Rather than remove the concrete, I just removed the rotted wood left embedded in it, leaving a post-shaped hole in a large, buried mass of concrete. The cavities were over 2 feet deep.
Rough redwood posts vary in their dimension, and the newer posts were slightly larger than the old, so I made adjustments accordingly. Once you start to slide the new post into the old hole, it has to fit right the first try. It gets wedged in quickly, and I treated each post with "copper-green" to retard rot and termites, so I wasn't going to be trimming these posts to fit after soaking them in poison.
The 2x4 rails are fitted into 2x4 holes notched into the posts. Here's how its done: layout the notch; score around it with a chisel; break out the top layer of wood with the chisel; plow out the rest with a drill and chainsaw...from both sides...
Here's a new post ready for the rail & fence sections to be re-fitted:
All finished: