Why we need re-import the ontology and give different default-graph-uri?

On the doc page, it says:

The base-iri and ontology-graph must NOT be the same as your default-graph-uri you entered at your salz adapter. If you delete it, because you want to update your ontology all data would be lost!

and

Navigate to Configuration WissKI Salz Adapters (WissKI Salz section), select the repository-id of the adapter you reimported the ontology and click on Compute Type and Property Hierarchy and Domains and Ranges, check Re-Compute results and start reasoning. You may see some errors of missing domains and ranges of some properties if you use Erlangen CRM / OWL. Check reasoning results if ontology is imported.

It seems there are some logic error of this sentence: If you delete it, because you want to update your ontology all data would be lost!

I cannot figure it out.