When I try to change the app url I get an error 400. Please advise.
Unfortunately, this is the same issue you have run into before. You have a revision outstanding which you closed/declined and it isn't being displayed again due to a bug in the UI. We just released a fix for this bug so you hopefully won't be able to get into this state again but because you have previously closed the revision notification you will have to work with developer relations to take the same steps you took last time.
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