Listed versions (including beta) can’t have an update_url. This is for security reasons, since listed versions are expected to be reviewed and monitored more closely by AMO reviewers.
I suggest you set up an install page for the new beta channel and then push an update to the AMO beta channel that asks users to switch to it. Inevitably some users will miss it and won’t move to the new channel, but hopefully the ones that are still interested in testing will find it.