Copy rows, remember old and new pkey

Hello list,

I need to make a copy of certain rows in table A and remember the
original pkey and new pkey in some temporary table B. Basically the copy
statement is INSERT INTO a SELECT * FROM a where a.x=y;

I guess I can do it with a plpgsql function and a FOR loop statement,
because I need two insert statements, the second using returned pkey
from first, but I was wondering if there's a simpler way, perhaps using
WITH?

FOR row IN SELECT * FROM a WHERE a.x=123 LOOP
INSERT INTO a (x, y, z) VALUES (row.x, row.y, row.z) RETURNING pkey
INTO new_pkey;
INSERT INTO b (old_id, new_id) VALUES (row.pkey, new_pkey);
END LOOP;

Thanks,

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