Point at and swap any value in an ltree. Kinda like the REPLACE() function.

postgres_ltree_swap.sql

CREATE OR REPLACE FUNCTION ltree_swap (_ltree ltree, _old_val LTREE, _new_val LTREE,  _position INT) RETURNS LTREE LANGUAGE plpgsql AS
$$
/*
==================================================
Author:     dvandenbosch
Created At: 11/14/2019
Use: SELECT '44.22.23'::TEXT::LTREE AS orginal, ltree_swap('44.22.23'::TEXT::LTREE,'23'::TEXT::LTREE,'fish'::TEXT::LTREE,2) AS swap
Note: _position parameter is zero based
==================================================
*/
    DECLARE
        --===========try catch vars===========
        _returned_sqlstate     TEXT;
        _message_text          TEXT;
        _pg_exception_detail   TEXT;
        _pg_exception_hint     TEXT;
        _pg_exception_context  TEXT;
        --====================================
        retval LTREE;
   BEGIN
    --try code goes here
IF nlevel(_ltree) = 0                       THEN  --empty ltree
    retval = ''::ltree;
ELSIF nlevel(_ltree)<(_position + 1)        THEN --Position out of scope for this particular ltree.
    retval = _ltree;                               --return orginal value
ELSIF nlevel(_ltree) = 1                    THEN --single value ltree
    IF _position = 0 AND _ltree = _old_val AND nlevel(_ltree) = 1 THEN
        retval = _new_val;
    ELSE
        retval = _ltree;
    END IF;
ELSIF nlevel(_ltree)=(_position + 1)     THEN --replace at the end
    raise notice 'bravo';
         SELECT COALESCE(foo.tree,_ltree) AS tree
    FROM (
        SELECT tree.tree
        FROM (
                 SELECT subpath(_ltree,0,(_position))  || _new_val AS tree
             ) AS tree(tree)
        WHERE 1=1
          AND subpath(_ltree,_position,1) = _old_val
          AND nlevel(_ltree) > _position
        UNION ALL
        SELECT NULL::LTREE
        ) AS foo
    LIMIT 1 INTO retval
      ;
ELSIF (nlevel(_ltree)>(_position + 1)) THEN
              SELECT COALESCE(foo.tree,_ltree) AS tree
    FROM (
        SELECT tree.tree
        FROM (
                 SELECT subpath(_ltree,0,(_position))  || _new_val  || subpath(_ltree,(_position+1))   AS  tree
             ) AS tree(tree)
        WHERE 1=1
          AND subpath(_ltree,_position,1) = _old_val
          AND nlevel(_ltree) > _position
        UNION ALL
        SELECT NULL::LTREE
        ) AS foo
    LIMIT 1 INTO retval
      ;
ELSE
    retval = 'senereo not determined'::TEXT::LTREE;
END IF;
   RETURN retval;
EXCEPTION WHEN others THEN
     GET STACKED DIAGNOSTICS
  _returned_sqlstate    = RETURNED_SQLSTATE
, _message_text         = MESSAGE_TEXT
, _pg_exception_detail  = PG_EXCEPTION_DETAIL
, _pg_exception_hint    = PG_EXCEPTION_HINT
, _pg_exception_context = PG_EXCEPTION_CONTEXT;
PERFORM it.pg_func_exceptions_insert (
                                          _returned_sqlstate
                                        , _message_text
                                        , _pg_exception_detail
                                        , _pg_exception_hint
                                        , _pg_exception_context
                                        , current_query()
                                        , FALSE  --false -> email will be sent to DBA upon function failure
                                       );
                 RAISE EXCEPTION E'Got exception:
            state  : %
            message: %
            detail : %
            hint   : %
            context: %', _returned_sqlstate, _message_text, _pg_exception_detail, _pg_exception_hint, _pg_exception_context;

   END;
 $$;