ALTER column TYPE varying question

I need to increase the length of a string field using version 8.1
I was thinking to use ALTER TABLE since now altering a column type
should be supported by pg.
The column is currently varying(60) and I want to have it varying(120)

After executing

ALTER TABLE mytable ALTER COLUMN mycolumn TYPE varchar(120)

I can see the column definition correctly changes and I can insert
rows with longer data in mycolumn.
But when I try to update data which were in the table before the ALTER
TABLE I get

"ERROR:  value too long for type character varying(60)"

It's like the old rows didn't update correctly keeping the old maximum length.

I can see there's an optional USING clause for the ALTER TYPE, but is
not really clear to me what should i add in this case, since basically
 I'd need no convertion...

Thanks

Paolo