1. Create a new table with the same fields the old one.
2. In the new table, add a field with Autonumber datatype.
3. Save it but do not open it nor write anything in there.
4. Create a select query with all fields of the old table.
5. Then, convert that query in append query and choose the new table.
6. Add each field to correspondent field, but the field with your old numbers put it in the Autonumber field of the new table.
7. Run the query.
8. That is all.
9. Then, Autonumber will continue with the following number.
10. Now you can put the Primary Key in Autonumber of the new table.