Is there a particular reason you need this? Generally, you would just have a Herald rule that would trigger every time a revision is submitted/updated, and nothing more.

Are you worried about putting to much work on your buildbots, or something?

EDIT: to make a Herald rule that triggers on every diff update, do this: Go to Herald, create a new Herald Rule. Create a rule for "Differential Revisions" that is "Global". On the conditions section, set the rule to:

"When all of these conditions are met: Repository, Is any of, <name here>"

The action is:

"Take these actions every time this rule matches: Run a build plan, <build plan name here>"

That's it, providing you have drydock configured correctly otherwise.

That's pretty much it.. triggering a new build every time we update the diff might seem a bit much

That being said, How can I trigger a build when a differential review has been updated? It's not obvious to me.

That's not currently possible - changing the status of a revision does not trigger herald.

It's also not usually desired, because you want the the tests to run before/while you review the code, so they can catch things before a human spends time on them.