Change SERIAL to INTEGER

Hi everybody:

In a computer which is un production, I have a PostgreSQL 7.4 database.
In a couple of tables, I have just realised that I made a small mistake:
the primary key is SERIAL but what I really need is an INTEGER, since I
do not need the auto-increment feature or the sequence. Until now, this
has not created any problem, since all INSERTs set the primary key
value, but I am not sure if in the future this could cause any problem.
I cannot drop and recreate the table or the column, since this column is
a used as foreign keys in other tables.

I have already tried to do the following:

ALTER TABLE mytable ALTER COLUMN mytableid DROP DEFAULT;
DROP SEQUENCE mytable_mytableid_seq;

but the last command fails, saying that mytable still uses this sequence.

Is there any way to change a SERIAL type to an INTEGER? I think that it
should be easy, since SERIAL is, in fact, an INTEGER with some conditions.

Thanks!

Pedro