Thanks! And just to be clear it would look something like this?

% #6653
\movedxref{algorithms.alg.nonmodifying.mismatch}{algorithms.alg.nonmodifying.alg.mismatch}

It's this example: \movedxref{old.label}{new.label}.
The old label is mismatch, and the new label alg.mismatch.

By the way, see https://github.com/cplusplus/draft/wiki/Commit-message-format.

Noted; I'm very much a rookie ;)

Hopefully the title of the PR is now fixed...

It should cover the commit message, not just the issue's title.
Also, the label should probably be [algorithms],
since you're changing both the [algorithm.syn] and [mismatch] within.

The label in the title?

In the commit message, according to the first example at https://github.com/cplusplus/draft/wiki/Commit-message-format#editorial-commits.

How important is it to change the commit messages? I can of course do so, but I gather that github strongly recommends against force pushing and I'm not aware of any other way.

PRs are generally a working space that are OK to force-push to.

I'll do this then :)

I'll prepend the first commit message with [algorithms]. But what about the second, i.e. the change to xrefdelta.tex?

I'll prepend the first commit message with [algorithms]. But what about the second, i.e. the change to xrefdelta.tex?

xrefdelta is just technical fall-out; you should ignore this for the commit message.

It's all part of the bigger change, so you can rebase to fixup.
That is, git rebase -i main, and change the second commit from pick to fixup.

Hopefully that's now done. Thank you so much for your help! (And of course I'm happy to make further changes if something isn't right.)