To increase the size of a subarea, insuring that all calc records will still be accessible, you would need to do an Unload-Reload to increase the size of the subarea. The original DMCL would be as is, and the new DMCL used by the unload-reload would need to have the new subarea definition. The original and new DMCLs would have to have different names so that both can be loaded during the Unload.
In this case the UNLOAD not only needs the New DMCL to calculate target pages, but also needs the new Subschema, because that is where the records are associated with a particular SUBAREA. The DMCL has no reference to specific records.
You will need to generate a new SS with a unique name and make it available
to the UNLOAD.