I must admit this has been a bit of a headache for me as well.
I just went https (smooth process, great) and decided to change the dated “out” email at the same time (basically gmail -> gandi). 4h later and umpteen rebuilds later I’m just reverting to the dated one (at least for today). 10 minutes per rebuild is not fun. Not to mention the time figuring out how to cleanup when I run out of space. I’ll be faster next time 
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Do all the settings in app.yml really require a rebuild? One would imagine changing email settings would just change something in one place.