Postgres 9.2 Change elements of a JSON object

If I were to perform the following script in newer versions of Postgres (13 and upward), then elements within the JSONB object can easily be added and amended using ||.

DO $$
declare j1 jsonb;
declare j2 jsonb;
declare j3 jsonb;
declare j4 jsonb;

begin

    j1 := ('{"A":'||false||',"B":'||false||',"C":'||false||',"D":'||false||',"E":'||false||',"F":'||false||'}')::jsonb;

    j2 := ('{"G":'||true||',"H":'||true||'}')::jsonb;
    j4 := j1||j2;
    raise notice '%',j4;
    --Required output: {"A":false,"B":false,"C":false,"D":false,"E":false,"F":false,"G":true,"H":true}

    j3 := ('{"A":'||false||',"B":'||true||',"C":'||false||',"D":'||false||',"E":'||true||',"F":'||false||'}')::jsonb;
    j4 := j4||j3;
    raise notice '%',j4;
    --Required output: {"A":false,"B":true,"C":false,"D":false,"E":true,"F":false,"G":true,"H":true}

end $$;

I am trying to achieve the same on Postgres 9.2. Since 9.2 is only compatible with JSON objects, I have to use JSON instead of JSONB. (Client not able to upgrade yet). However, the same code for JSON gives errors.

DO $$
declare j1 json;
declare j2 json;
declare j3 json;
declare j4 json;

begin

    j1 := ('{"A":'||false||',"B":'||false||',"C":'||false||',"D":'||false||',"E":'||false||',"F":'||false||'}')::json;

    j2 := ('{"G":'||true||',"H":'||true||'}')::json;
    j4 := j1||j2;
    raise notice '%',j4;
    --Required output: {"A":false,"B":false,"C":false,"D":false,"E":false,"F":false,"G":true,"H":true}

    j3 := ('{"A":'||false||',"B":'||true||',"C":'||false||',"D":'||false||',"E":'||true||',"F":'||false||'}')::json;
    j4 := j4||j3;
    raise notice '%',j4;
    --Required output: {"A":false,"B":true,"C":false,"D":false,"E":true,"F":false,"G":true,"H":true}

end $$;

Are there any functions that I can use to add/amend elements in a JSON object in Postgres 9.2? Ultimately achieving the same results in the second block of code as in the first block of code. Thanks.