changing multiple pk's in one update

Hello all,

I have a table with a primary key column
that contains sequential numbers.

Sometimes I need to shift them all up or down
by a fixed amount. For example, if I have
four rows with primary keys, 2, 3, 4, 5, I
might want to shift them down by 1 by doing:

UPDATE mytable SET id=id-1

(where "id" is the pk column) so that the pk's
are now 1, 2, 3, 4.

When I try to shift them up by using +1 in the
above update statement, I get (not surprisingly)
a duplicate key error. I also realize that the
-1 case above works only by luck.

So my question:
Is there some way, perhaps with ORDER BY, that
I can achieve the change I want with a single
update statement? (If I have an unused key
range large enough, I suppose I could update
all the keys to that range, and then back to
my target range but the requires two updates
(there are a lot of foreign keys referencing
these primary keys) and requires that I have
an available range, so a single update statement
would be preferable.)

Thanks for any enlightenment.