Best practice for making changes to "old" branches

[Adam Lipscombe]

Folks,


Forgive me if this a no-brainer. I am not a CVS guru.

We have a system of branching before a release.
The branch is then tested, fixed, tested, fixed (however many times is
necessary) and then the release is produced.

Meanwhile ongoing development continues in the head.

When the branch is released, it is merged back into the head so that any
fixes are propagated.



My problem is that recently a customer discovered a big bug in a release
that was produced from a branch that is "3 branches ago". i.e. it has long
been merged into the head.

We could retrieve the branch code and fix it, then produce a new release.
Or we could fix it in the latest branch and try to persuade the cust to take
the latest release version (they would not want to 'cos they are hot on
acceptance testing)



If we fix it in their original branch, how do we propagate the change into
the head? 
(The branch has already been merged in?)


It occurs to me that this must be a standard problem. What is best practice?


TIA -Adam