hstore to json and back again

Hello,

I'm trying to migrate an existing hstore column to json in Postgres
9.3, and I'd like to be able to run the script in reverse. I know not
all JSON can turn back into hstore, but since this is coming from an
old hstore column, I know the structure is flat (no nesting), and that
all values are strings.

Here is the SQL I'm using to go hstore -> json:

UPDATE foo
SET datahash_new = to_json(datahash_old)
;

Is there any SQL I can use to go backwards?:
UPDATE foo
SET datahash_old = xxxxx(datahash_new)
;

I understand why there is not a general-purpose solution, but in my
case this should be possible. I've tried to cook something up with
json_each_text, but I haven't been able to figure it out. Can anyone
offer any help?

Thanks,
Paul

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Pulchritudo splendor veritatis.