We tore up our carpet and padding, and in looking under the big cold air
return grate, I noticed that we have two floors; not just 2 hardwood floors,
I'm talking hardwood, then the wood subfloor, then joists, then under THAT
is another "subfloor," or what is the ceiling in the basement, then more
joists under that.

My vote is to sand and refinish the existing floor, but we may be getting
brand new hardwood, which I believe means I need to tear up the old hardwood
floor. I just wanted to see if anyone knows why I have these two complete
floors, and if there's anything I need to be concerned about or be cautious
of when potentially ripping up the old floor.

I have PICTURES and questions that relate to them. If the URL's below
aren't links you can click on, just copy/paste the URL into your browser.

http://www.grillof.com/_images/AirReturn.jpg

The above is a picture looking into the cold air return. As you can see,
the Sub Floor is coming in from the wall diagonally, running across the
seams of the top hardwood floor. HOWEVER, look at the following picture in
the SAME cold air return:

http://www.grillof.com/_images/StraightSub.jpg

In this picture, you see that the same sub floor that is diagonal pieces
coming in from the wall is actually STRAIGHT pieces after only a couple
inches in from the wall. The surface you see under the subflkoor is just
some plank that's covering something, I don't know what yet. Look at the
right of the pic; those pieces run diagonally, you can see from the seams
between pieces. Now look at the top of the air return hole; those pieces
run parallel with the top hardwood pieces! Does this indicate that the
subfloor USED to be diagonal, but then someone cut it all up and replaced it
with straight subfloor, leaving the diagonal pieces jutting out a little
from the sides?

Now here's the view from the basement steps, looking back up towards the
top; it shows the level of the floor upstairs, and then the joists that you
can see in the basement (you can see from the distance between upstairs'
floor and the ceiling joists in the basement that the basement joists are
not the same ones seen through the cold air return upstairs):

http://www.grillof.com/_images/FloorLevels.jpg

Now, in the last pic, I'm looking up at the ceiling in the basement. You
can see the cold air return duct on the left, the diagonal pieces that make
the ceiling (NOT the subfloor of the upstairs), and the joists that can be
seen from the basement:

http://www.grillof.com/_images/BasementCeiling.jpg

So, does anyone know why I have two sets of joists and subfloor, one on top
of the other? I've been to look at 3 old houses around town, and they just
have a floor, subfloor, and joists, and the joists are the same ones you see
in the basement. In my house, I have a floor, subfloor, joists, then more
floor, then more joists. The house was built in 1929, and is plaster
walls/ceilings, if that helps anything.

So, to sum up:

1) Why do I see diagonal subfloor pieces that stop and are replaced with
straight-running subfloor pieces upstairs? And does this add serious
complications to the idea of tearing up the top hardwood floor?

2) Why do I have a "double" floor, like a set of two complete floor
assemblies (subfloor and joists on top of subfloor and joists)?

3) Does anything described and seen in the pics mean that I'm in for a
world of hurt in terms of tearing up hardwood and putting in new?

Thanks, everyone, for reading, for your input and thoughts, advice,
cautions, etc.

Complete Newb