In order to do just what you want, the complete "answer" needs to be known. You may know what the complete answer is, but this has not yet been posted. I believe that you want to do will have issues because the generated values will not follow a managable sequence (i.e. which values will be higher or lower than others.
What about the values that already exist? What you say you want to do will cause existing entries to be "out of sequence".
Suggest you consider either increasing the size of the field or keep this size and make the field a packed-decimal or binary field.
Hope this helps,