downloads
  order     Name   dlid  website
  0         Jon    51     A
  1         Jim    54     A
  2         Jack   55     A
  5         Bob    67     B
  12        Adam   70     A
  20        Aron   76     A
  21        James  90     A
  32        Jay    100    B
  ...

update downloads set "order" =
    case
        when 'order' = 0 then "order" = 5468
        when "order" < 5468 then "order" + 5468
    end
where website = 'A';

downloads
  order     Name   dlid  website
  5         Bob    67     B
  32        Jay    100    B
  ...
  5468      Jon    51     A
  5469      Jim    54     A
  5470      Jack   55     A
  5480      Adam   70     A
  5488      Aron   76     A
  5489      James  90     A

downloads
order     Name   dlid  website
5         Bob    67     B
32        Jay    100    B
...
5468      Jon    51     A
5469      Jim    54     A
5470      Jack   55     A
5471      Adam   70     A
5472      Aron   76     A
5473      James  90     A