To change the update channel of an already-built install, adjust $APPDIR/Contents/Resources/defaults/pref/channel-prefs.js
to use pref("app.update.channel", $CHANNEL_NAME);
However, the cert used is baked into the updater based on channel name. Ideally we switch this channel name before the try staging release, or we may have to patch the updater cert (see below).
Essentially, if the try staging run has an update channel of, say, beta
, the updater will expect the release mar cert but the MARs will be signed with the dep cert.