Yes that will be some cleanup with the same layer in dozens of different services and items.
That is interesting, I will try that exact experiment: if the popup has not been 'customized' and ALL fields are visible, what happens, when new fields are added to the service? Will they magically show up and be visible by default? I will test that tomorrow. Because typically the 'internal' fields we would want to hide across the board so I could just hide those on the services themselves. That is good to know that domain changes came through automatically, will also test that.
I also recall seeing something about where to save JSON for popups, either in the map or from the service. Like when I went to save a map with New Map Viewer and I recall it asking that I believe. That would definitely be a way around this whole thing. If we could just automate republishing (or restarting; with schema lock disabled) all services, if the webmap can pull popups from the Service, I think that would solve it. For the life of me I can't find that option though, I think it was in New map viewer. I know Pro warns about that when overwriting. Does anyone know where that option is, to choose where to save popups for an existing webmap? I can't seem to find it.
Regarding copying the config to Word yes I have been using AGOL Assistant for a long time and copy and pasting in JSON, I used to do that to fix layer IDs before unique layer IDs were possible. But to do that across many webmaps would be time consuming and error-prone, just like reconfiguring many popups manually. So, thank you Laura... that might be a solution. To make all fields visible. If it works. Make all fields visible, and leave it that way and see if new fields are added and also visible, automatically. (one would think the API would have like 'all fields visible' as a property, vs hard-coding explicitly all that JSON; that'd be a good feature enhancement too)