Peter:

Excuse me for butting in with a couple of observations/suggestions:

(i) The invoice number is not going to be the primary key. What restraints (if any) do you want to place upon this column?
(ii) You said that you already have data in the table. (That is a familiar situation.) Put this table to one side and create a new table from scratch, structuring it the way you want it to be. (One of its columns will be the identity column.) Then import data in from the original table using DTS and SQL's autoincrement to populate the rows.

Godfrey