Fail to update sub-org level DefaultCurrency via API

We are using multi-suborg mechanism in CLZ.
And trying to override organization.DefaultCurrency property per sub org.

But the api fails like this : 
[InvalidOperation] Invalid operation: Field 'DefaultCurrency' cannot be updated. The field is marked 'read-only'. (Reference Id: h61C2xe1rfQVoadagr2gd)

How can I override sub organisation fields via API? 
Isnt it possible ?