PL/SQL dynamic update error

I have a table with informations that change every month, and I have to
update the monthly data in a new column. To do that, I try to find out wich
months are being trated and I perform an update in my table for every month.

So what I'm trying to do is to change the month column for a fixed name,
perform my selects and updates in the renamed column and change the name
back for its old value. Then I do exactly the same thing for another month.
It would be something like this:

FOR month IN select month from monht_values LOOP
v_command: = 'ALTER TABLE my_table
RENAME COLUMN m'
||month
||' TO month_value;';

EXECUTE v_command;

UPDATE my_table
SET month_value = new_value
WHERE key = my_key;

v_command: = 'ALTER TABLE my_table
RENAME COLUMN month_value TO m'
||month
||';';
EXECUTE v_command;

END LOOP;

The query works just fine, but the updates are made allways in the same
column. Even when I change the name, PostgreSQL doesn't recognize it, and
all the updates are performed in the same first column name. It seems like
it stores the column information in a way I can't change the it.

Any ideas on how to fix it?