Can't get git rebase --onto to work

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