I am not sure wheather it is designed intentionally, so I just present this fact.
If a mosaic is created which has the same name and properties with previous one, the describetion of the mosaic can be updated, even the mosaic has been sent.
I try this on the test net. The mosaic name is algas:algea.
This is by design.
"Modifying a Mosaic
The NEM API has the best explanation about this, and it is copied verbatim here:
There might be the need to alter a mosaic definition, either because you want to change the description or because you supplied faulty properties or faulty levy data. This is done simply by issuing another mosaic definition creation transaction as described above with the same mosaic id but different description/properties/levy. However there are some restriction when doing so:
The description can be changed at any point even if the creator does not own the entire supply. Properties and the levy data can only be changed if the creator owns every single mosaic of that type. This is necessary to prevent the creator from secretly introducing a levy or inflating the mosaic by increasing the supply. Keep in mind that renewing the mosaic definition costs you the creation fee again, so it is worthwhile to double check the data before issuing the transaction."
https://rb2nem.github.io/nem-dev-guide/09-mosaics/
