Need to change old commit (and regenerate tree)

Hi all

My question is this: I have two branches (say B and C) where one is
reachable from the other (say B is ancestor of C), and if they are
separately rebased about/onto the same point, why do B and C become
non-overlapping branches?

Let me explain with an example.

Say I have the following commit line:

A--------o--------o--------o--------B--------o--------o--------C

A, B and C are branches (so that B is reachable from C, and A is
reachable from B). [For ease, I am drawing the branches at the same
level since there are no real diverging branches here.]

I then realise that I want to change the commit A and have both B and
C rebased on this changed commit.

Now when I do a
  $ git rebase --onto A2 A C

This results in two parallel trees like these:

A--------o--------o--------o--------B--------o--------o--------o(old C)

A2--------o--------o--------o--------o--------o--------o--------C

Now I go about rebasing B. I can of course 'reset' B to C~3. But
alternatively, if I decide to do a rebase:
  $ git rebase --onto A2 A B

I will end up getting

A--------o--------o--------o--------o(old B)--------o--------o--------o(old C)

A2--------o--------o--------o--------o--------o--------o--------C
   \
     ` -----o--------o--------o--------B

Instead of (what I initially expected):

A--------o--------o--------o--------o(old B)--------o--------o--------o(old C)

A2--------o--------o--------o--------B--------o--------o--------C


So what I am missing here? Aren't the new commits B~1, B~2, B~3
identical to C~4, C~5, C~6 (respectively) in all ways so as to have
gotten them the same SHA1 and hence appear as what I expected them to
appear?

I have taken a simple example here. In reality, I wanted to change a
not so new commit (on the main line), and there were many branches
diverging out from the main line after the (bad) commit. I initially
thought I could just write a simple script that would rebase all
branches that have the bad commit as an ancestor, and the new tree
would be a mirror image of the original. But that was not to be! There
is also the problem of resetting tags (and possible notes).


I am sorry if this has already been discussed, please point me to the
right resource if so.

My git version:
  $ git --version
  git version 1.7.0.5

Thanks
Antriksh Pany