And I would still claim that this is easier in MongoDB because several versions of the phone number field(s) can happily coexist in the same collection. Those v

And I would still claim that this is easier in MongoDB because several versions of the phone number field(s) can happily coexist in the same collection. Those variants are usually trivial to understand for someone who even just looks at the data, and the application can be written to either accept the different formats, or adjust the format on the fly when it encounters a document that still uses an old schema. Or you could indeed write a batch job that bumps all your phone numbers to a new format, and you could put a JSON schema constraint on your collection that enforces the new schema for every future document. All those possibilities exist, and I truly see that as a big advantage.