Can't get git rebase --onto to work

[Howard Miller <howard@e-learndesign.co.uk>]

Trying to explain this as concisely as possible.

I started with the following branches (names changed to protect the guilty)...

* clientA
* clientB

both have a common ancestry....

I then checked out clientB created a new branch clientB_patch and did
a load of work and commits.

However, I actually wanted all those commits to apply to clientA
branch instead so....

git checkout clientA
git checkout -b clientA_patch     (to ensure I didn't wreck original branch)
git rebase --onto clientA_patch clientB clientB_patch

It says "rewinding" head and then applies the commits I would have expected.

However, I have now been switched to the 'clientB_patch' branch and it
says that it differs from its tracked remote branch by hundreds of
commits.

clientA_patch is unchanged. It seems to have mangled clientB_patch and
done nothing to clientA_patch. Where did I go wrong?

TIA