convert from json to text[]

Hi,

I am running postgres 9.2.4 and 1 of my columns (column name is old_field)
is type json. When I select from this table, I get the following sample
data

select old_field from table1;

[],
['a', 'b']
['a']
[]

How do I change the data type from json to text[]?

I have tried

alter table table1 add column new_field text[] default '{}'::text[]"
UPDATE table1 SET new_field = {old_field} where old_field is not null"

Questions:
1) how to I change the data type from json to text[]
2) how to I select rows that has data and not [],

thanks
-m