Change character set in domain definition

Hello ,

I need to change the character set of certain domains. Below I have
included an example of the current DDL and what it should be.
However in the documentation there is no mention howto do this.

Is this possible?

----currently defined as-----
CREATE DATABASE 'Probos:C:\WorkArea\SURAPON.GDB'
USER 'SYSDBA' PASSWORD 'masterkey'
PAGE_SIZE 4096
DEFAULT CHARACTER SET ISO8859_1;

CREATE DOMAIN DMNULL_BIGSTRING AS
VARCHAR(80) CHARACTER SET NONE;

---Should be----
CREATE DOMAIN DMNULL_BIGSTRING AS
VARCHAR(80);

TIA
Paul Sjoerdsma