Hi Z,
What David Lerwill suggest works, but it's not the only way. One problem with it is having to rebuild all relationships onto the new table.
Another way would be to make a copy, empty the original, then change the field type twice: once into a normal number(long integer), then back into Autonum (you can only do that on an empty table, as you'll have noticed). Then you can make either a query or a VBA-procedure thats adds the records from the copied table, with new numbers in place of the original autonum-value. This relies on the funny feature that you can add records with autonumber-values of your own choice as long as you explicitly assign a value to that field:
INSERT INTO [tablename] ( autonumfieldname ) SELECT valueyouchoose AS autonumfieldname;
It's a bit of work, especially if you have enforced integrity in your relationships: you'll need to drop the enforcement temporarily and update all related values to the changed values in the source table.
If you need it bad enough, you'll be able to pull it off.
Greetings, good luck, Daan