(I will bold my questions hereafter, so that they are easy to notice)
Thanks, what I did was indeed creating some new visualizations, exporting them, inspecting the ndjson, finding the reference field(s), copying the content and pasting inside the reference array (as suggested in the managing-saved-objects-in-kibana-7.7 guide) in the edit object view in Kibana. Much like what is suggested in change-visualization-index-under-saved-objects.
Modifying the index field under kibanaSavedObjectMeta.searchSourceJSON as suggested in visualizations-broken-after-deleting-index-pattern-resolved-the-index-pattern-associated-with-this-object-no-longer-exists is something which apparently is valid for kibana 6.8, judging by the part at the end in the guide managing-saved-objects-in-kibana-6.8.
However I still had to put my index's uuid in the reference, which visualizations-broken-after-deleting-index-pattern-resolved-the-index-pattern-associated-with-this-object-no-longer-exists suggests to take from the link to the index page under index management in kibana.
My problem was that I didn't have a simple single-object and easy-to-guess reference that could be solved just with a new index uuid. For example, I had a Control kind of visualization that required 3 entries in the reference array, each with a custom name. And I couldn't have guessed the exact required structure and naming just by looking at what is provided in the Edit Visualization view (Saved Objects > Inspect) alone, since when I edit objects (also valid and new ones!) from there, the reference entry is always empty.
I don't see why it should be like that, since when one then goes the more laborious way of downloading the export.ndarray, the reference can be found.
Why isn't all the information present in Kibana's view for editing objects?
Anyway, my question remains:
is there a way of debugging such issues?
Unfortunately, when a visualization is broken, it just doesn't show up (can't be visualized), and it does so silently. Even the guide managing-saved-objects-in-kibana-7.7 warns you at the end about the fact that even a little change might render the visualization unusable, and that manual editing should be avoided:
Validation is not performed for object properties. Submitting an invalid change will render the object unusable.
But what about the cases when I must edit manually? Is there a way to discover if it's a wrong type in one field, if it's wrong syntax, if it's a missing index's uuid, or wrong number of entries or format in the reference or in other fields?
And a final question related to this topic, which I ask in advance so that it won't bite me:
if I now introduce index aliases (I need rollover on my index), or change index name or index pattern name, or I change some field's meta-information (like index or type or keyword), what's the correct and cleanest way of propagating this change to my visualizations?
Can I do something in advance? (For example, I don't see any button or field to change the index pattern when I edit a visualization in the proper "UI" way)
Or will I have to go through this process of blind trial and error via manual editing of the saved object? (Now I know how to do it but it's a bit time consuming and error-prone).
Thanks for your support and thank you again for your quick answers!