need magic to shuffle some numbers

Hi,

there is a table that has among others a integer primary key "id" and 
another integer column "prio" as well as an integer "group_id".

I'd like to invert the values of the prio-column for one of the groups.
The prio numbers start with 3 and there are 1159 different prios in this 
group.
At least every value appeares only once.   :)

Is there an elegant way to switch the prio values around so that every 
record with the first prio gehts the last and vice versa?
Then the records with the second smallest prio get the second-to-last 
biggest value and v.v.
...


regards