According to Frank J Warner :
Storm blew down a portion of a 30-year-old good-neighbor fence. 6 foot
high, 4x4 redwood posts set in concrete, 2x4 rails, 1x6 dog-ear cedar
panels. Surprised it lasted this long.

We're rebuilding the entire fence and have a couple of questions.

First, should we re-use the old post holes? We'd dig out the existing
concrete and pour new, but the soil is sandy and not too stable to
begin with. Digging the old concrete out will undoubtedly leave holes
too big to fill economically unless we use cylindrical concrete forms.
OTOH, moving the post holes over a few feet to undisturbed soil will
create unsightly "short" panels at either end of the fence.

If you really can't move the posts, and want to stick with concrete,
sonotube is the way to go, plus backfill.

[Tho, even if the hole is excessively wide, a pour of the same
amount of concrete into the hole you'd have used for a "minimum"
post hole will hold things up almost as well. Less likely to split
too.]

Before pouring the concrete, bed the bottom few inches of the post
in gravel, with a few inches of gravel below the post bottom.

[prevents rot, allows the water to drain out of the post.]

If you're burying the posts deep enough (ie: 4'+ in these parts),
you may not need the concrete at all except on posts with severe
tipping force (ie: gate posts).

Second, does anybody recommend setting PT 4x4 posts in galvanized metal
post brackets embedded into the concrete rather than directly into the
concrete? I see those beefy brackets at HD and wonder if they would
actually hold up in a windy environment like mine. The age of the old
fence tells me that setting the posts directly into the concrete works
just as well if not better.

I don't. The post brackets I'm aware of are really only suitable
for deck posts, where the post is prevented from tipping by deck
structure.

The brackets I'm familiar with won't stand up to trying to hold
a freestanding 6' "solid" (ie: privacy) fence in strong winds.
Too much leverage over too short a length of post. They'll
eventually break. Or destroy the bottom of the post and the post
will snap off.

[This includes the "deck spike" type of brackets (the top of the
"spike" is a square "socket" for the post) as well as more
traditional "saddle" brackets.]
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.