David WE Roberts wrote:

This leaves an ugly hole about a foot deep and very wide.
What I want in the same location is an 8" wide hole 18" deep, to take
the new post and the concrete around it.

Do they *have* to be in the same location? As in, couldn't you site the
first post a couple of feet back along the fence... that way all the new
posts would be offset against the positions of the old ones, which you
wouldn't need to remove, just chop them flush with the ground[1]

David

[1]But not using a circular saw blade on an angle grinder... (qv)