change varchar(10) to varchar()

is there a sql command to change a field data type
from varchar(10) to varchar()?

i tried to do this in pgadmin3, but when i deleted the
length (maximum characters) attribute, the OK button
stayed grayed out and inactive.  as an aside, the OK
button becomes active when the length attribute is
changed...  but turns gray when it is deleted
altogether.

i don't want to put any arbitrary constraints on my
s/n field - especially since there is no performance
benefit in doing so per the manual.

tia...

ps - i can delete and recreate the table with no
problems - but i would prefer to learn something for
future reference, if possible.

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