Hi!
Then what happens with the "unit":"1" that currently we have in JSON? It seems that one cannot enter this string in the field, and I guess it would (rightly) be invalid as a unit. Will Wikibase continue to use this (invalid) string as a placeholder for "no unit"?
We could make a bot to convert those to Q199. RDF export already does it. Of course, not all of them are actually unit-less values, some of them may be something that actually needs a unit, so we could wait for a while to give people a chance to fix that and then convert the remaining ones to Q199.
If every wiki needs to define its own unit items to see labels, how is data exchange supposed to work? Will the RDF export then contain a different IRI for "meter" when exporting data from Wikidata and from Commons (or whatever other future instance)?
That's a very interesting point, I think this can be handled by establishing: 1. Types/classes of units, such as "measure of length" 2. Designated standard unit - e.g. "meter" is a "standard measure of length". 3. Conversion properties - e.g. "foot" is 0.305 "meters" 4. Having RDF exports contain values converted to standardized measures - i.e. every quantity that has unit that is "measure of length" will also have a value expressed in "meters".
Then reconciling data between instances would be just reconciling properties for the above (which can be made easier by finding some existing ontology featuring units and relating to it), and then matching entities for standard measures.