declare
    column_exists exception;
    pragma exception_init (column_exists , -00001);
begin
    execute immediate 'UPDATE EMPRESA_IMPOSTO SET CODIGOCONTABILMUNICIPAL = NULL';
    exception when column_exists then null;
end;
/
declare
    column_exists exception;
    pragma exception_init (column_exists , -01430);
begin
    execute immediate 'ALTER TABLE EMPRESA_IMPOSTO MODIFY (CODIGOCONTABILMUNICIPAL VARCHAR2(15))';
    exception when column_exists then null;
end;