properly sequenced autonumber field"
Aye, theres the rub. What if rows have been deleted?
If this is a one-time-only operation, you could ensure the ID field has no missing values by appending the table to a new table which has the same columns plus one more autonumber column, TempID. The extra column will be filled as the rows are inserted and there wont be any missing values. Use the TempID column as PHV says to flag the rows.
Now you have an extra table which you can use henceforth, or you could UPDATE the new column in the original table based on a JOIN. Which also leads to the thought that all you need in the temporary table is a foreign key, that is, a column with the values from the primary key column in the original table.
Bet you are thinking why should something so easy be so difficult? Fundamentally, the rows in a relational database table do not have any order. The idea of every 6th record has no meaning in SQL.