Update new column with value from old columns for all the rows
The solution is very simple.  If you do 

UPDATE exchange
SET emp_id = {some expression here on the params column}; -- no WHERE clause!


it will do exactly what you want: pick the params from the row to be updated, calculate the expression and update emp_id with its value.  As there is no WHERE clause, it will go through the whole table.  You don't need the ID of the row or anything else.